記述言語：英語   掲載種別：研究論文（学術雑誌）  

The Hall effect plays a significant role in star formation, because it induces rotation in the infalling envelope, which in turn affects the formation and evolution of the circumstellar disc. The importance of the Hall effect varies with the Hall coefficient, and this coefficient is determined by the fractional abundances of charged species. These abundance values are primarily based on the size and quantity of dust grains as well as the cosmic ray intensity, which, respectively, absorb and create charged species. Thus, the Hall coefficient varies with both the properties of dust grains and the cosmic ray rate (or ionization source). In this study, we explore the dependence of the Hall coefficient on the grain size and cosmic ray ionization rate using a simplified chemical network model. Following this, using an analytic model, we estimate the typical size of a circumstellar disc induced solely by the Hall effect. The results show that the disc grows during the main accretion phase to a size of ∼3-100 au, with the actual size depending on the parameters. These findings suggest that the Hall effect greatly affects circumstellar disc formation, especially in the case that the dust grains have a typical size of ∼0.025 − 0.075 μm.

DOI： 10.1093/mnras/sty3524

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The evolution of collapsing clouds embedded in different star-forming environments is investigated using three-dimensional non-ideal magnetohydrodynamics simulations considering different cloud metallicities ($\mathrm{\mathit{ Z}}/\, \mathrm{Z}-\odot$ = 0, 10^-5, 10^-4, 10^-3, 10^-2, 10^-1, and 1) and ionization strengths (C_ζ = 0, 0.01, 1, and 10, where C_ζ is a coefficient controlling the ionization intensity and C_ζ = 1 corresponds to the ionization strength of nearby star-forming regions). With all combinations of these considered values of $\mathrm{\mathit{ Z}}/\, \mathrm{Z}-\odot$ and C_ζ, 28 different star-forming environments are prepared and simulated. The cloud evolution in each environment is calculated until the central density reaches $n\approx 10^{16}\, {
m cm}^{-3}$ just before protostar formation, and the outflow driving conditions are derived. An outflow appears when the (first) adiabatic core forms in a magnetically active region where the magnetic field is well coupled with the neutral gas. In cases where outflows are driven, their momentum fluxes are always comparable to the observations of nearby star-forming regions. Thus, these outflows should control the mass growth of the protostars as in the local universe. Roughly, an outflow appears when $\mathrm{\mathit{ Z}}/\, \mathrm{Z}-\odot \gt 10^{-4}$ and C_ζ ≥ 0.01. It is expected that the transition of the star formation mode from massive stars to normal solar-type stars occurs when the cloud metallicity is enhanced to the range of $\mathrm{\mathit{ Z}}/\, \mathrm{Z}-\odot \approx 10^{-4}$-10^-3, above which relatively low-mass stars would preferentially appear as a result of strong mass ejection.

DOI： 10.1093/mnras/stz1079

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The effect of misalignment between the magnetic field B and the angular momentum Jang of molecular cloud cores on the angular momentum evolution during the gravitational collapse is investigated by ideal and non-ideal MHD simulations. For the non-ideal effect, we consider the ohmic and ambipolar diffusion. Previous studies that considered the misalignment reported qualitatively contradicting results. Magnetic braking was reported as being either strengthened or weakened by misalignment in different studies. We conducted simulations of cloud core collapse by varying the stability parameter α (the ratio of the thermal to gravitational energy of the core) with and without including magnetic diffusion. The non-ideal MHD simulations show the central angular momentum of the core, with θ=0° (Jang B) being always greater than that with θ=90° (Jang B), independently of α, meaning that circumstellar disks form more easily in a core with θ=0°. The ideal MHD simulations, in contrast, show the central angular momentum of the core with θ=90° being greater than with θ=0° for small α and smaller for large α. Inspection of the angular momentum evolution of the fluid elements reveals three mechanisms contributing to the evolution of the angular momentum: (i) magnetic braking in the isothermal collapse phase, (ii) selective accretion of the rapidly (for θ = 90°) or slowly (for θ = 0°) rotating fluid elements to the central region, and (iii) magnetic braking in the first core and the disk. The difference between the ideal and non-ideal simulations arises from the different efficiencies of (iii).

DOI： 10.3847/1538-4357/aae4dc

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the submillimeter continuum condensation SMM4 in Serpens Main using the Atacama Large Millimeter/submillimeter Array during its Cycle 3 in 1.3 mm continuum, ¹²CO J = 2-1, SO J _N = 6₅-5₄, and C¹⁸O J = 2-1 lines at angular resolutions of ∼0.″55 (240 au). The 1.3 mm continuum emission shows that SMM4 is spatially resolved into two protostars embedded in the same core: SMM4A showing a high brightness temperature, 18 K, with little extended structure and SMM4B showing a low brightness temperature, 2 K, with compact and extended structures. Their separation is ∼2100 au. Analysis of the continuum visibilities reveals a disk-like structure with a sharp edge at r ∼ 240 au in SMM4A, and a compact component with a radius of 56 au in SMM4B. The ¹²CO emission traces fan-shaped and collimated outflows associated with SMM4A and SMM4B, respectively. The blue and red lobes of the SMM4B outflow have different position angles by ∼30°. Their inclination and bending angles in the 3D space are estimated at i _b ∼ 36°, i _r ∼ 70°, and ∼ 40°, respectively. The SO emission traces shocked regions, such as cavity walls of outflows and the vicinity of SMM4B. The C¹⁸O emission mainly traces an infalling and rotating envelope around SMM4B. The C¹⁸O fractional abundance in SMM4B is ∼50 times smaller than that of the interstellar medium. These results suggest that SMM4A is more evolved than SMM4B. Our studies in Serpens Main demonstrate that continuum and line observations at millimeter wavelengths allow us to differentiate evolutionary phases of protostars within the Class 0 phase.

DOI： 10.3847/1538-4357/aacf9b

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report ALMA Cycle 3 observations in CO isotopes toward a dense core, MC27/L1521F in Taurus, which is considered to be at an early stage of multiple star formation in a turbulent environment. Although most of the high-density parts of this core are considered to be as cold as ∼10 K, high-angular resolution (∼20 au) observations in ¹²CO (J = 3-2) revealed complex warm (>15-60 K) filamentary/clumpy structures with the sizes from a few tens of astronomical units to ∼1000 au. The interferometric observations of ¹³CO and C¹⁸O show that the densest part with arc-like morphologies associated with the previously identified protostar and condensations are slightly redshifted from the systemic velocity of the core. We suggest that the warm CO clouds may be consequences of shock heating induced by interactions among the different density/velocity components that originated from the turbulent motions in the core. However, such a small-scale and fast turbulent motion does not correspond to a simple extension of the line-width-size relation (i.e., Larson's law), and thus the actual origin remains to be studied. The high-angular resolution CO observations are expected to be essential in detecting small-scale turbulent motions in dense cores and to investigate protostar formation therein.

DOI： 10.3847/1538-4357/aac898

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The driving mechanism of massive outflows observed in high-mass star-forming regions is investigated using three-dimensional magnetohydrodynamics (MHD) and protostellar evolution calculations. In our previous paper, we showed that the mass outflow rate depends strongly on the mass accretion rate on to the circumstellar disc around a high-mass protostar, and massive outflows may be driven by the magnetic effect in high-mass star-forming cores. In this study, in order to verify that the MHD disc wind is the primary driving mechanism of massive outflows, we quantitatively compare outflow properties obtained through simulations and observations. Since the outflows obtained through simulations are slightly younger than those obtained through observations, the time-integrated quantities of outflow mass, momentum, and kinetic energy are slightly smaller than those obtained through observations. On the other hand, time-derivative quantities of mass ejection rate, outflow momentum flux, and kinetic luminosity obtained through simulations are in very good agreement with those obtained through observations. This indicates that the MHD disc wind greatly contributes to the massive outflow driving from high-mass protostars, and the magnetic field might significantly control the high-mass star formation process.

DOI： 10.1093/mnras/stx3070

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Star formation in magnetically subcritical clouds is investigated using a three-dimensional non-ideal magnetohydrodynamic simulation. Since rapid cloud collapse is suppressed until the magnetic flux is sufficiently removed from the initially magnetically subcritical cloud by ambipolar diffusion, it takes ≳5-10 t_ff to form a protostar, where t_ff is the freefall time-scale of the initial cloud. The angular momentum of the star-forming cloud is efficiently transferred to the interstellar medium before the rapid collapse begins, and the collapsing cloud has a very low angular momentum. Unlike the magnetically supercritical case, no large-scale lowvelocity outflow appears in such a collapsing cloud due to the short lifetime of the first core. Following protostar formation, a very weak high-velocity jet, which has a small momentum and might disappear at a later time, is driven near the protostar, while the circumstellar disc does not grow during the early mass accretion phase. The results show that the star formation process in magnetically subcritical clouds is qualitatively different from that in magnetically supercritical clouds.

DOI： 10.1093/mnras/stx2589

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report ALMA observations in 0.87 mm continuum and ¹²CO (J = 3-2) toward a very low-luminosity (<0.1 L _o) protostar, which is deeply embedded in one of the densest cores, MC27/L1521F, in Taurus with an indication of multiple star formation in a highly dynamical environment. The beam size corresponds to ∼20 au, and we have clearly detected blueshifted/redshifted gas in ¹²CO associated with the protostar. The spatial/velocity distributions of the gas show there is a rotating disk with a size scale of ∼10 au, a disk mass of ∼10^-4 M _o, and a central stellar mass of ∼0.2 M _o. The observed disk seems to be detached from the surrounding dense gas, although it is still embedded at the center of the core whose density is ∼10⁶ cm^-3. The current low-outflow activity and the very low luminosity indicate that the mass accretion rate onto the protostar is extremely low in spite of a very early stage of star formation. We may be witnessing the final stage of the formation of ∼0.2 M _o protostar. However, we cannot explain the observed low luminosity with the standard pre-main-sequence evolutionary track unless we assume cold accretion with an extremely small initial radius of the protostar (∼0.65 ). These facts may challenge our current understanding of the low mass star formation, in particular the mass accretion process onto the protostar and the circumstellar disk.

DOI： 10.3847/1538-4357/aa8e9e

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have recently observed the Class 0/I protostar L1527 IRS using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 1 in 220 GHz dust continuum and C¹⁸O line emissions with a ∼2 times higher angular resolution and ∼4 times better sensitivity than our ALMA Cycle 0 observations. Continuum emission shows elongation perpendicular to the associated outflow, with a deconvolved size of C¹⁸O emission shows similar elongation, indicating that both emissions trace the disk and the flattened envelope surrounding the protostar. The velocity gradient of the C¹⁸O emission along the elongation due to rotation of the disk/envelope system is reanalyzed, identifying Keplerian rotation proportional to more clearly than the Cycle 0 observations. The Keplerian-disk radius and the dynamical stellar mass are kinematically estimated to be ∼74 au and , respectively. The continuum visibility is fitted by models without any annulus averaging, revealing that the disk is in hydrostatic equilibrium. The best-fit model also suggests a density jump by a factor of ∼5 between the disk and the envelope, suggesting that disks around protostars can be geometrically distinguishable from the envelope from a viewpoint of density contrast. Importantly, the disk radius geometrically identified with the density jump is consistent with the kinematically estimated radius. Possible origin of the density jump due to the mass accretion from the envelope to the disk is discussed. C¹⁸O observations can be reproduced by the same geometrical structures derived from the dust observations, with possible C¹⁸O freeze-out and localized C¹⁸O desorption.

DOI： 10.3847/1538-4357/aa8264

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the discovery of an extremely young protostar, SMM11, located in the associated submillimeter condensation in the Serpens Main cluster using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 3 at 1.3 mm and an angular resolution of . SMM11 is a Class 0 protostar without any counterpart at 70 μm or shorter wavelengths. The ALMA observations show 1.3 mm continuum emission associated with a collimated ¹²CO bipolar outflow. Spitzer and Herschel data show that SMM11 is extremely cold ( 26 K) and faint ( 0.9 ). We estimate the inclination angle of the outflow to be , almost parallel to the plane of the sky, from simple fitting using a wind-driven-shell model. The continuum visibilities consist of Gaussian and power-law components, suggesting a spherical envelope with a radius of ∼600 au around the protostar. The estimated low C¹⁸O abundance, X(C¹⁸O) = 1.5-3 , is also consistent with its youth. The high outflow velocity, a few 10 at a few 1000 au, is much higher than theoretical simulations of first hydrostatic cores, and we suggest that SMM11 is a transitional object right after the second collapse of the first core.

DOI： 10.3847/2041-8213/aa9701

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The relation between the mass accretion rate on to the circumstellar disc and the rate of mass ejection by magnetically driven winds is investigated using three-dimensional magnetohydrodynamics simulations. Using a spherical cloud core with a varying ratio of thermal to gravitational energy, which determines the mass accretion rate on to the disc, to define the initial conditions, the outflow propagation for approximately 10⁴ yr after protostar formation is then calculated for several cloud cores. The mass ejection rate and accretion rate are comparable only when the magnetic energy of the initial cloud core is comparable to the gravitational energy. Consequently, in strongly magnetized clouds a higher mass accretion rate naturally produces bothmassive protostars and massive outflows. The simulated outflowmass, momentum, kinetic energy and momentum flux agree well with observations, indicating that massive stars form through the same mechanism as low-mass stars but require a significantly strong magnetic field to launch massive outflows.

DOI： 10.1093/mnras/stx893

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the formation of circumstellar disks and outflows subsequent to the collapse of molecular cloud cores with the magnetic field and turbulence. Numerical simulations are performed by using an adaptive mesh refinement to follow the evolution up to ∼1000 years after the formation of a protostar. In the simulations, circumstellar disks are formed around the protostars; those in magnetized models are considerably smaller than those in nonmagnetized models, but their size increases with time. The models with stronger magnetic fields tend to produce smaller disks. During evolution in the magnetized models, the mass ratios of a disk to a protostar is approximately constant at ∼1%-10%. The circumstellar disks are aligned according to their angular momentum, and the outflows accelerate along the magnetic field on the 10-100 au scale; this produces a disk that is misaligned with the outflow. The outflows are classified into two types: a magnetocentrifugal wind and a spiral flow. In the latter, because of the geometry, the axis of rotation is misaligned with the magnetic field. The magnetic field has an internal structure in the cloud cores, which also causes misalignment between the outflows and the magnetic field on the scale of the cloud core. The distribution of the angular momentum vectors in a core also has a non-monotonic internal structure. This should create a time-dependent accretion of angular momenta onto the circumstellar disk. Therefore, the circumstellar disks are expected to change their orientation as well as their sizes in the long-term evolutions.

DOI： 10.3847/1538-4357/aa6a1c

記述言語：英語   掲載種別：研究論文（学術雑誌）  

One of the outstanding problems in star formation theory concerns the transfer of angular momentum so that mass can accrete onto a newly born young stellar object (YSO). From a theoretical standpoint, outflows and jets are predicted to play an essential role in the transfer of angular momentum 1,2,3,4 and their rotations have been reported for both low-5 and high-mass 6,7 YSOs. However, little quantitative discussion on outflow launching mechanisms has been presented for high-mass YSOs due to a lack of observational data. Here we present a clear signature of rotation in the bipolar outflow driven by Orion Source I, a high-mass YSO candidate, using the Atacama Large Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon monoxide (Si 18 O) reveals a velocity gradient perpendicular to the outflow axis, which is consistent with that of the circumstellar disk traced by a high excitation water line. The launching radii and outward velocity of the outflow are estimated to be >10 au and 10 km s â '1, respectively. These parameters rule out the possibility that the observed outflow is produced by the entrainment of a high-velocity jet 8, and that contributions from the stellar wind 9 or X-wind 10, which have smaller launching radii, are significant in the case of Source I. Thus these results provide convincing evidence of a rotating outflow directly driven by the magneto-centrifugal disk wind launched by a high-mass YSO candidate 6,11.

DOI： 10.1038/s41550-017-0146

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We study formation and long-term evolution of a circumstellar disk in a collapsing molecular cloud core using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues, and its radius becomes as large as 200 au toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum in the highly resistive disk. Although the spiral arms disappear in a few rotations as expected in a classical theory, new spiral arms form recurrently as the disk, soon becoming unstable again by gas accretion. Such recurrent spiral arms persist throughout the Class-0 and I phases. We then perform synthetic observations and compare our model with a recent high-resolution observation of a young stellar object Elias 2-27, whose circumstellar disk has grand-design spiral arms. We find good agreement between our theoretical model and the observation. Our model suggests that the grand-design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

DOI： 10.3847/2041-8213/835/1/L11

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We perform three-dimensional radiation non-ideal magnetohydrodynamics simulations and investigate the impact of the Hall effect on the angular momentum evolution in collapsing cloud cores in which the magnetic field B and angular momentum J_ang are misaligned with each other. We find that the Hall effect noticeably changes the magnetic torques in the pseudo-disk, and strengthens and weakens the magnetic braking in cores with acute and obtuse relative angles between B and J_ang, respectively. This suggests that the bimodal evolution of the disk size may occur in the early disk evolutionary phase even if B and J_ang are randomly distributed. We show that a counter-rotating envelope forms in the upper envelope of the pseudo-disk in cloud coreswith obtuse relative angles. We also find that a counter-rotating region forms at the midplane of the pseudo-disk in cloud cores with acute relative angles. The former and latter types of counter-rotating envelopes may be associated with young stellar objects with large (r ~ 100 au) and small (r ≤ 10 au) disks, respectively.

DOI： 10.1093/pasj/psx113

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have measured continuum flux densities of a high-mass protostar candidate, a radio source I in the Orion KL region (Orion Source I) using the Atacama Large Millimeter/Submillimeter Array (ALMA) at band 8 with an angular resolution of 0.″1. The continuum emission at 430, 460, and 490 GHz associated with Source I shows an elongated structure along the northwest-southeast direction perpendicular to the so-called low-velocity bipolar outflow. The deconvolved size of the continuum source, 90 au ×20 au, is consistent with those reported previously at other millimeter/submillimeter wavelengths. The flux density can be well fitted to the optically thick blackbody spectral energy distribution, and the brightness temperature is evaluated to be 700-800 K. It is much lower than that in the case of proton-electron or H^- free-free radiations. Our data are consistent with the latest ALMA results by Plambeck & Wright, in which the continuum emission was proposed to arise from the edge-on circumstellar disk via thermal dust emission, unless the continuum source consists of an unresolved structure with a smaller beam filling factor.

DOI： 10.3847/1538-4357/833/2/238

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The angular momentum distribution and its redistribution are of crucial importance in the formation and evolution of circumstellar discs. Many molecular line observations towards young stellar objects indicate that the radial distributions of the specific angular momentum j have a characteristic profile. In the inner region, typically R ≲ 100 au, the specific angular momenta distribute as j ∝ r^1/2, indicating the existence of a rotationally supported disc. In the outer regions, R ≳ 5000 au, j increases as the radius increases, and the slope is steeper than unity. This behaviour is assumed to reflect the original angular momentum distributions in the maternal molecular clouds. In the intermediate region, 100 au ≲ R ≲ 5000 au, the jdistribution appears to be almost flat. While this is often interpreted to be a consequence of the conservation of the specific angular momentum, the interpretation actually is insufficient and requires a stronger condition that the initial distribution of j must be spatially uniform. However, this requirement seems to be unrealistic and inconsistent with observations. In this work, we propose a simple alternative explanation: the apparently flat j profile is produced by strong elongation owing to the large velocity gradient in the accreting flow, no matter what the initial j-distribution is. In order to show this, we provide a simple analytic model for the gravitational collapse of molecular clouds. We also propose a method to estimate the ages of protostars using only the observed rotation profile. We demonstrate the validity of this method in comparison with hydrodynamic simulations, and apply the model to the young stellar objects L1527 IRS, TMC-1A and B335.

DOI： 10.1093/MNRAS/STW1994

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We present the results of ALMA observations of dust continuum emission and molecular rotational lines toward a dense core MC27 (aka L1521F) in Taurus, which is considered to be at a very early stage of star formation. The detailed column density distributions on size scales from a few tens to ∼10,000 AU are revealed by combining the ALMA (12 m array + 7 m array) data with the published/unpublished single-dish data. The high angular resolution observations at 0.87 mm with a synthesized beam size of ∼0.″74 × 0.″32 reveal that a protostellar source, MMS-1, is not spatially resolved and lacks associated gas emission, while a starless high-density core, MMS-2, has substructures in both dust and molecular emission. The averaged radial column density distribution of the inner part of MC27/L1521F (r ≲ 3000 AU) is ∼ , clearly flatter than that of the outer part, ∼. The complex velocity/spatial structure obtained with previous ALMA observations is located inside the inner flatter region, which may reflect the dynamical status of the dense core.

DOI： 10.3847/0004-637X/826/1/26

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Disc formation in strongly magnetized cloud cores is investigated using a three-dimensional magnetohydrodynamic simulation with a focus on the effects of the initial cloud stability and the mass accretion rate. The initial cloud stability greatly alters the disc formation process even for prestellar clouds with the same mass-to-flux ratio. A high mass accretion rate on to the disc-forming region is realized in initially unstable clouds, and a large angular momentum is introduced into the circumstellar region in a short time. The region around the protostar has both a thin infalling envelope and a weak magnetic field, which both weaken the effect of magnetic braking. The growth of the rotation-supported disc is promoted in such unstable clouds. Conversely, clouds in an initially near-equilibrium state show lower accretion rates of mass and angular momentum. The angular momentum is transported to the outer envelope before protostar formation. After protostar formation, the circumstellar region has a thick infalling envelope and a strong magnetic field that effectively brakes the disc. As a result, disc formation is suppressed when the initial cloud is in a nearly stable state. The density distribution of the initial cloud also affects the disc formation process. Disc growth strongly depends on the initial conditions when the prestellar cloud has a uniform density, whereas there is no significant difference in the disc formation process in prestellar clouds with non-uniform densities.

DOI： 10.1093/mnras/stw2256

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the Class I protostar TMC-1A with the Atacama Millimeter/submillimeter Array (ALMA) in the emissions of ¹²CO and C¹⁸O (J = 2-1) and 1.3 mm dust continuum. Continuum emission with a deconvolved size of 0.″50 × 0.″37, perpendicular to the ¹²CO outflow, is detected. It most likely traces a circumstellar disk around TMC-1A, as previously reported. In contrast, a more extended structure is detected in C¹⁸O, although it is still elongated with a deconvolved size of 3.″3 × 2.″2, indicating that C¹⁸O traces mainly a flattened envelope surrounding the disk and the central protostar. C¹⁸O shows a clear velocity gradient perpendicular to the outflow at higher velocities, indicative of rotation, while an additional velocity gradient along the outflow is found at lower velocities. The radial profile of the rotational velocity is analyzed in detail, finding that it is given as a power law ∝r^-a with an index of ∼0.5 at higher velocities. This indicates that the rotation at higher velocities can be explained as Keplerian rotation orbiting a protostar with a dynamical mass of 0.68 (inclination corrected). The additional velocity gradient of C¹⁸O along the outflow is considered to be mainly infall motions in the envelope. Position-velocity diagrams made from models consisting of an infalling envelope and a Keplerian disk are compared with the observations, revealing that the observed infall velocity is ∼0.3 times smaller than the free-fall velocity yielded by the dynamical mass of the protostar. Magnetic fields could be responsible for the slow infall velocity. A possible scenario of Keplerian disk formation is discussed.

DOI： 10.1088/0004-637X/812/1/27

記述言語：英語  

DOI： 10.1088/2041-8205/810/2/L26

記述言語：英語  

DOI： 10.1093/mnras/stv1290

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The main accretion phase of star formation is investigated in clouds with different metallicities in the range 0 ≤ Z ≤ Z⊙, resolving the protostellar radius. Starting from a near-equilibrium prestellar cloud, we calculate the cloud evolution up to ˜100 yr after the first protostar forms. Star formation differs considerably between clouds with lower (Z ≤ 10-4 Z⊙) and higher (Z > 10-4 Z⊙) metallicities. Fragmentation frequently occurs and many protostars appear without a stable circumstellar disc in lower-metallicity clouds. In these clouds, although protostars mutually interact and some are ejected from the cloud centre, many remain as a small stellar cluster. In contrast, higher-metallicity clouds produce a single protostar surrounded by a nearly stable rotation-supported disc. In these clouds, although fragmentation occasionally occurs in the disc, the fragments migrate inwards and finally fall on to the central protostar. The difference in cloud evolution is due to different thermal evolutions and mass accretion rates. The thermal evolution of the cloud determines the emergence and lifetime of the first core. The first core develops prior to the formation of a protostar in higher-metallicity clouds, whereas no (obvious) first core appears in lower-metallicity clouds. The first core evolves into a circumstellar disc with a spiral pattern, which effectively transfers the angular momentum outwards and suppresses frequent fragmentation. In lower-metallicity clouds, the higher mass accretion rate increases the disc surface density within a very short time, rendering the disc unstable to self-gravity and inducing vigorous fragmentation.

DOI： 10.1093/mnras/stu2633

その他リンク： http://ads.nao.ac.jp/abs/2015MNRAS.448.1405M

記述言語：英語  

DOI： 10.1088/0004-637X/801/2/117

記述言語：英語  

DOI： 10.1093/mnras/stu2160

記述言語：英語  

DOI： 10.1088/0004-637X/796/2/131

記述言語：英語  

DOI： 10.1088/2041-8205/796/1/L17

記述言語：英語  

DOI： 10.1088/0004-637X/793/1/1

記述言語：英語  

DOI： 10.1088/2041-8205/789/1/L4

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10-2-106 m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

DOI： 10.1088/0004-637X/784/2/109

その他リンク： http://ads.nao.ac.jp/abs/2014ApJ...784..109T

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation of a circumstellar disc in collapsing cloud cores is investigated with three-dimensional magnetohydrodynamic simulations. We prepare four types of initial cloud having different density profiles and calculate their evolution with or without a sink. To investigate the effect of magnetic dissipation on disc formation, Ohmic dissipation is considered in some models. Calculations show that disc formation is very sensitive to both the initial cloud configuration and the sink treatment. The disc size considerably differs in clouds with different density profiles even when the initial clouds have almost the same mass-to-flux ratio. Only a very small disc (˜10 au in size) appears in clouds with a uniform density profile, whereas a large disc (˜100 au in size) forms in clouds with a Bonnor-Ebert density profile. In addition, a large sink accretion radius numerically impedes disc formation during the main accretion phase and tends to foster the misleading notion that disc formation is completely suppressed by magnetic braking. The protostellar outflow is also greatly affected by the sink properties. A sink accretion radius of ≲1 au and sink threshold density of ≳1013 cm-3 are necessary for investigating disc formation during the main accretion phase.

DOI： 10.1093/mnras/stt2343

その他リンク： http://ads.nao.ac.jp/abs/2014MNRAS.438.2278M

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

We report the results of our three-dimensional radiation hydrodynamics simulation of collapsing unmagnetized molecular cloud cores. We investigate the formation and evolution of the circumstellar disc and the clumps formed by disc fragmentation. Our simulation shows that disc fragmentation occurs in the early phase of circumstellar disc evolution and many clumps form. The clump can be represented by a polytrope sphere of index n ˜ 3 and n ≳ 4 at central temperature Tc ≲ 100 K and Tc ≳ 100 K, respectively. We demonstrate, numerically and theoretically, that the maximum mass of the clump, beyond which it inevitably collapses, is ˜0.03 M⊙. The entropy of the clump increases during its evolution, implying that evolution is chiefly determined by mass accretion from the disc rather than by radiative cooling. Although most of the clumps rapidly migrate inward and finally fall on to the protostar, a few clumps remain in the disc. The central density and temperature of the surviving clump rapidly increase and the clump undergoes a second collapse within 1000-2000 years after its formation. In our simulation, three second cores of masses 0.2 M⊙, 0.15 M⊙ and 0.06 M⊙ formed. These are protostars or brown dwarfs rather than protoplanets. For the clumps to survive as planetary-mass objects, the rapid mass accretion should be prevented by some mechanisms.

DOI： 10.1093/mnras/stt1684

その他リンク： http://ads.nao.ac.jp/abs/2013MNRAS.436.1667T

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation of Population III stars is investigated using resistive magnetohydrodynamic simulations. Starting from a magnetized primordial prestellar cloud, we calculate the cloud evolution several hundreds of years after first protostar formation, resolving the protostellar radius. When the natal minihalo field strength is weaker than B ≲ 10-13(n/1 cm-3)-2/3 G (n is the hydrogen number density), magnetic effects can be ignored. In this case, fragmentation occurs frequently and a stellar cluster forms, in which stellar mergers and mass exchange between protostars contribute to the mass growth of these protostars. During the early gas accretion phase, the most massive protostar remains near the cloud centre, whereas some of the less massive protostars are ejected. The magnetic field significantly affects Population III star formation when B ≳ 10-12(n/1 cm-3)-2/3 G. In this case, because the angular momentum around the protostar is effectively transferred by both magnetic braking and protostellar jets, the gas falls directly on to the protostar without forming a disc, and only a single massive star forms. In addition, a massive binary stellar system appears when 10- 13(n/1 cm- 3)- 2/3 ≲ B ≲ 10- 12(n/1 cm- 3)- 2/3 G. Therefore, the magnetic field determines the end result of the formation process (cluster, binary or single star) for Population III stars. Moreover, no persistent circumstellar disc appears around the protostar regardless of the magnetic field strength, which may influence the further evolution of Population III stars.

DOI： 10.1093/mnras/stt1524

その他リンク： http://ads.nao.ac.jp/abs/2013MNRAS.435.3283M

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the formation process of self-gravitating protoplanetary disks in unmagnetized molecular clouds. The angular momentum is redistributed by the action of gravitational torques in the massive disk during its early formation. We develop a simplified one-dimensional accretion disk model that takes into account the infall of gas from the envelope onto the disk and the transfer of angular momentum in the disk with an effective viscosity. First we evaluate the gas accretion rate from the cloud core onto the disk by approximately estimating the effects of gas pressure and gravity acting on the cloud core. We formulate the effective viscosity as a function of the Toomre Q parameter that measures the local gravitational stability of the rotating thin disk. We use a function for viscosity that changes sensitively with Q when the disk is gravitationally unstable. We find a strong self-regulation mechanism in the disk evolution. During the formation stage of protoplanetary disks, the evolution of the surface density does not depend on the other details of the modeling of effective viscosity, such as the prefactor of the viscosity coefficient. Next, to verify our model, we compare the time evolution of the disk calculated with our formulation with that of three-dimensional hydrodynamical simulations. The structures of the resultant disks from the one-dimensional accretion disk model agree well with those of the three-dimensional simulations. Our model is a useful tool for the further modeling of chemistry, radiative transfer, and planet formation in protoplanetary disks.

DOI： 10.1088/0004-637X/770/1/71

その他リンク： http://ads.nao.ac.jp/abs/2013ApJ...770...71T

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

The evolution of protostellar outflow is investigated with resistive magneto-hydrodynamic nested-grid simulations that cover a wide range of spatial scales (˜1 au-1 pc). We follow cloud evolution from the pre-stellar core stage until the infalling envelope dissipates long after the protostar formation. We also calculate protostellar evolution to derive protostellar luminosity with time-dependent mass accretion through a circumstellar disc. The protostellar outflow is driven by the first core prior to protostar formation and is directly driven by the circumstellar disc after protostar formation. The opening angle of the outflow is large in the Class 0 stage. A large fraction of the cloud mass is ejected in this stage, which reduces the star formation efficiency to ˜50 per cent. After the outflow breaks out from the natal cloud, the outflow collimation is gradually improved in the Class I stage. The head of the outflow travels more than ˜105 au in ˜105 yr. The outflow momentum, energy and mass derived in our calculations agree well with observations. In addition, our simulations show the same correlations among outflow momentum flux, protostellar luminosity and envelope mass as those in observations. These correlations differ between Class 0 and I stages, which are explained by different evolutionary stages of the outflow; in the Class 0 stage, the outflow is powered by the accreting mass and acquires its momentum from the infalling envelope; in the Class I stage, the outflow enters the momentum-driven snow-plough phase. Our results suggest that protostellar outflow should determine the final stellar mass and significantly affect the early evolution of low-mass protostars.

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

We investigate the formation and evolution of circumstellar discs in turbulent cloud cores until several 104 yr after protostar formation using smoothed particle hydrodynamics (SPH) calculations. The formation and evolution process of circumstellar disc in turbulent cloud cores differs substantially from that in rigidly rotating cloud cores. In turbulent cloud cores, a filamentary structure appears before the protostar formation and the protostar forms in the filament. If the turbulence is initially sufficiently strong, the remaining filament twists around the protostar and directly becomes a rotation-supported disc. Upon formation, the disc orientation is generally misaligned with the angular momentum of its host cloud core and it dynamically varies during the main accretion phase, even though the turbulence is weak. This is because the angular momentum of the entire cloud core is mainly determined by the large-scale velocity field whose wavelength is comparable to the cloud scale, whereas the angular momentum of the disc is determined by the local velocity field where the protostar forms and these two velocity fields do not correlate with each other. In the case of disc evolution in a binary or multiple stars, the discs are misaligned with each other at least during the main accretion phase, because there is no correlation between the velocity fields around the position where each protostar forms. In addition, each disc is also misaligned with the binary orbital plane. Such misalignment can explain the recent observations of misaligned discs and misaligned protostellar outflows.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the first three-dimensional radiation magnetohydrodynamic (RMHD) simulations of protostellar collapse with and without Ohmic dissipation. We take into account many physical processes required to study star formation processes, including a realistic equation of state. We follow the evolution from molecular cloud cores until protostellar cores are formed with sufficiently high resolutions without introducing a sink particle. The physical processes involved in the simulations and adopted numerical methods are described in detail. We can calculate only about one year after the formation of the protostellar cores with our direct three-dimensional RMHD simulations because of the extremely short timescale in the deep interior of the formed protostellar cores, but successfully describe the early phase of star formation processes. The thermal evolution and the structure of the first and second (protostellar) cores are consistent with previous one-dimensional simulations using full radiation transfer, but differ considerably from preceding multi-dimensional studies with the barotropic approximation. The protostellar cores evolve virtually spherically symmetric in the ideal MHD models because of efficient angular momentum transport by magnetic fields, but Ohmic dissipation enables the formation of the circumstellar disks in the vicinity of the protostellar cores as in previous MHD studies with the barotropic approximation. The formed disks are still small (less than 0.35 AU) because we simulate only the earliest evolution. We also confirm that two different types of outflows are naturally launched by magnetic fields from the first cores and protostellar cores in the resistive MHD models.

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

The configuration and evolution of the magnetic field in star-forming cores are investigated in order to directly compare simulations and observations. We prepare four different initial clouds having different magnetic field strengths and rotation rates, in which magnetic field lines are aligned/misaligned with the rotation axis. First, we calculate the evolution of such clouds from the prestellar stage until long after protostar formation. Then, we calculate the polarization of thermal dust emission expected from the simulation data. We create polarization maps with arbitrary viewing angles and compare them with observations. Using this procedure, we confirmed that the polarization distribution projected on the celestial plane strongly depends on the viewing angle of the cloud. Thus, by comparing the observations with the polarization map predicted by the simulations, we can roughly determine the angle between the direction of the global magnetic field and the line of sight. The configuration of the polarization vectors also depends on the viewing angle. We find that an hourglass configuration of magnetic field lines is not always realized in a collapsing cloud when the global magnetic field is misaligned with the cloud rotation axis. Depending on the viewing angle, an S-shaped configuration of the magnetic field (or the polarization vectors) appears early in the protostellar accretion phase. This indicates that not only the magnetic field but also the cloud rotation affects the dynamical evolution of such a cloud. In addition, by comparing the simulated polarization with actual observations, we can estimate properties of the host cloud such as the evolutionary stage, magnetic field strength, and rotation rate.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We measured polarized dust emission at 350 μm toward the high-mass star-forming massive dense clump IRAS 20126+4104 using the SHARC II Polarimeter, SHARP, at the Caltech Submillimeter Observatory. Most of the observed magnetic field vectors agree well with magnetic field vectors obtained from a numerical simulation for the case when the global magnetic field lines are inclined with respect to the rotation axis of the dense clump. The results of the numerical simulation show that rotation plays an important role on the evolution of the massive dense clump and its magnetic field. The direction of the cold CO 1-0 bipolar outflow is parallel to the observed magnetic field within the dense clump as well as the global magnetic field, as inferred from optical polarimetry data, indicating that the magnetic field also plays a critical role in an early stage of massive star formation. The large-scale Keplerian disk of the massive (proto)star rotates in an almost opposite sense to the clump's envelope. The observed magnetic field morphology and the counterrotating feature of the massive dense clump system provide hints to constrain the role of magnetic fields in the process of high-mass star formation.

DOI： 10.1088/2041-8205/750/2/L29

その他リンク： http://ads.nao.ac.jp/abs/2012ApJ...750L..29S

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The effects of a protostellar outflow on the star formation in a single cloud core are investigated by three-dimensional resistive magnetohydrodynamic (MHD) simulations. Starting from the pre-stellar cloud core, the star formation process is calculated until the end of the main accretion phase. In the calculations, the mass of the pre-stellar cloud is parametrized. During the star formation, the protostellar outflow is driven by the circumstellar disc. The outflow extends also in the transverse direction until its width becomes comparable to the initial cloud scale, and thus the outflow has a wide opening angle of ≳40°. As a result, the protostellar outflow sweeps up a large fraction of the infalling material and ejects it into the interstellar space. The outflow can eject at most over half of the host cloud mass, significantly decreasing the star formation efficiency. The outflow power is stronger in clouds with a greater initial mass. Thus, the protostellar outflow effectively suppresses the star formation efficiency in a massive cloud. The outflow weakens significantly and disappears in several free-fall time-scales of the initial cloud after the cloud begins to collapse. The natal pre-stellar core influences the lifetime and size of the outflow. At the end of the main accretion phase, a massive circumstellar disc comparable in mass to the protostar remains. Calculations show that ˜26-54 per cent of the initial cloud mass is converted into the protostar and ˜8-40 per cent remains in the circumstellar disc, while ˜8-49 per cent can be ejected into the interstellar space by the protostellar outflow. Therefore, the protostellar outflow can decrease the star formation efficiency to ˜50 per cent at the maximum.

DOI： 10.1111/j.1365-2966.2011.20336.x

その他リンク： http://ads.nao.ac.jp/abs/2012MNRAS.421..588M

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate gas accretion flow onto a circumplanetary disk from a protoplanetary disk in detail by using high-resolution three-dimensional nested-grid hydrodynamic simulations, in order to provide a basis of formation processes of satellites around giant planets. Based on detailed analyses of gas accretion flow, we find that most of gas accretion onto circumplanetary disks occurs nearly vertically toward the disk surface from high altitude, which generates a shock surface at several scale heights of the circumplanetary disk. The gas that has passed through the shock surface moves inward because its specific angular momentum is smaller than that of the local Keplerian rotation, while gas near the midplane in the protoplanetary disk cannot accrete to the circumplanetary disk. Gas near the midplane within the planet's Hill sphere spirals outward and escapes from the Hill sphere through the two Lagrangian points L1 and L2. We also analyze fluxes of accreting mass and angular momentum in detail and find that the distributions of the fluxes onto the disk surface are well described by power-law functions and that a large fraction of gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times the Hill radius. The nature of power-law functions indicates that, other than the outer edge, there is no specific radius where gas accretion is concentrated. These source functions of mass and angular momentum in the circumplanetary disk would provide us with useful constraints on the structure and evolution of the circumplanetary disk, which is important for satellite formation.

DOI： 10.1088/0004-637X/747/1/47

その他リンク： http://ads.nao.ac.jp/abs/2012ApJ...747...47T

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We performed hydrodynamical simulations to investigate the formation and evolution of protostars and circumstellar discs from the pre-stellar cloud. As the initial state, we adopted the molecular cloud core with two non-dimensional parameters representing the thermal and rotational energies. With these parameters, we derived 17 models and calculated the cloud evolution ˜104 yr after the protostar formation. We found that early evolution of the star-disc system can be qualitatively classified into four modes: the massive-disc, early-fragmentation, late-fragmentation, and protostar-dominant modes. In the 'massive-disc mode', to which the majority of models belong, the disc mass is greater than the protostellar mass for over 104 yr and no fragmentation occurs in the circumstellar disc. The collapsing cloud shows fragmentation before the protostar formation in the 'early-fragmentation mode'. The circumstellar disc shows fragmentation after the protostar formation in the 'late-fragmentation mode', in which the secondary star gains most of its mass from the circumstellar disc after fragmentation and has a mass comparable to that of the primary star. The protostellar mass rapidly increases and exceeds the circumstellar disc mass in the 'protostar-dominant mode'. This mode appears only when the initial molecular cloud core has a very small rotational energy. Comparison of our results with observations indicates that the majority of protostars have a fairly massive disc during the main accretion phase: the circumstellar disc mass is comparable to or more massive than the protostar mass. It is expected that such a massive disc promotes gas-giant formation by gravitational instability in a subsequent evolutionary stage.

DOI： 10.1111/j.1365-2966.2011.19081.x

その他リンク： http://ads.nao.ac.jp/abs/2011MNRAS.416..591T

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using resistive magnetohydrodynamics simulation, we consider circumstellar disk formation in a strongly magnetized cloud. As the initial state, an isolated cloud core embedded in a low-density interstellar medium with a uniform magnetic field was adopted. The cloud evolution was calculated until almost all gas inside the initial cloud fell onto either the circumstellar disk or a protostar, and a part of the gas was ejected into the interstellar medium by the protostellar outflow driven by the circumstellar disk. In the early main accretion phase, the disk size is limited to ˜10 AU because the angular momentum of the circumstellar disk is effectively transferred by both magnetic braking and the protostellar outflow. In the later main accretion phase, however, the circumstellar disk grows rapidly and exceeds ≳ 100 AU by the end of the main accretion phase. This rapid growth of the circumstellar disk is caused by depletion of the infalling envelope, while magnetic braking is effective when the infalling envelope is more massive than the circumstellar disk. The infalling envelope cannot brake the circumstellar disk when the latter is more massive than the former. In addition, the protostellar outflow weakens and disappears in the later main accretion phase, because the outflow is powered by gas accretion onto the circumstellar disk. Although the circumstellar disk formed in a magnetized cloud is considerably smaller than that in an unmagnetized cloud, a circumstellar disk exceeding 100 AU can form even in a strongly magnetized cloud.

その他リンク： http://ads.nao.ac.jp/abs/2011PASJ...63..555M

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：日本語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

記述言語：英語   掲載種別：研究論文（学術雑誌）  

掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We perform a long-term simulation of star and disk formation using three-dimensional nonideal magnetohydrodynamics. The simulation starts from a prestellar cloud and proceeds through the long-term evolution of the circumstellar disk until ∼1.5 × 105 yr after protostar formation. The disk has size ≲50 au and little substructure in the main accretion phase because of the action of magnetic braking and the magnetically driven outflow to remove angular momentum. The main accretion phase ends when the outflow breaks out of the cloud, causing the envelope mass to decrease rapidly. The outflow subsequently weakens as the mass accretion rate also weakens. While the envelope-to-disk accretion continues, the disk grows gradually and develops transient spiral structures, due to gravitational instability. When the envelope-to-disk accretion ends, the disk becomes stable and reaches a size ≳300 au. In addition, about 30% of the initial cloud mass has been ejected by the outflow. A significant finding of this work is that after the envelope dissipates a revitalization of the wind occurs, and there is mass ejection from the disk surface that lasts until the end of the simulation. This mass ejection (or disk wind) is generated because the magnetic pressure significantly dominates both the ram pressure and thermal pressure above and below the disk at this stage. Using the angular momentum flux and mass-loss rate estimated from the disk wind, the disk dissipation timescale is estimated to be ∼106 yr.

DOI： 10.3847/1538-4357/ad4997

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出版者・発行元：Monthly Notices of the Royal Astronomical Society: Letters  

The detection of Keplerian rotation is rare among Class 0 protostellar systems. We have investigated the high-density tracer HCN as a probe of the inner disc in a Class 0 proto-brown dwarf candidate. Our ALMA high angular resolution observations show the peak in the HCN (3-2) line emission arises from a compact component near the proto-brown dwarf with a small bar-like structure and a deconvolved size of ∼50 au. Radiative transfer modelling indicates that this HCN feature is tracing the innermost, dense regions in the proto-brown dwarf where a small Keplerian disc is expected to be present. The limited velocity resolution of the observations, however, makes it difficult to confirm the rotational kinematics of this feature. A brightening in the HCN emission towards the core centre suggests that HCN can survive in the gas phase in the inner, dense regions of the proto-brown dwarf. In contrast, modelling of the HCO+ (3-2) line emission indicates that it originates from the outer pseudo-disc/envelope region and is centrally depleted. HCN line emission can reveal the small-scale structures and can be an efficient observational tool to study the inner disc properties in such faint compact objects where spatially resolving the disc is nearly impossible.

DOI： 10.1093/mnrasl/slae044

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記述言語：英語  

DOI： 10.1002/smtd.202301163

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PubMed

出版者・発行元：Astrophysical Journal  

We report Atacama Large Millimeter/submillimeter Array/Atacama Compact Array observations of a high-density region of the Corona Australis cloud forming a young star cluster, and the results of resolving internal structures. In addition to embedded Class 0/I protostars in the continuum, a number of complex dense filamentary structures are detected in the C18O and SO lines by the 7 m array. These are substructures of the molecular clump that are detected by the total power array as extended emission. We identify 101 and 37 filamentary structures with widths of a few thousand astronomical units in C18O and SO, respectively, which are called feathers. The typical column density of the feathers in C18O is about 1022 cm−2, and the volume density and line mass are ∼105 cm−3 and a few M ☉ pc−1, respectively. This line mass is significantly smaller than the critical line mass expected for cold and dense gas. These structures have complex velocity fields, indicating a turbulent interior. The number of feathers associated with Class 0/I protostars is only ∼10, indicating that most of them do not form stars but rather are transient structures. The formation of feathers can be interpreted as a result of colliding gas flow because the morphology is well reproduced by MHD simulations, and this is supported by the presence of H i shells in the vicinity. The colliding gas flows may accumulate gas and form filaments and feathers, and trigger the active star formation of the R CrA cluster.

DOI： 10.3847/1538-4357/ad40a6

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

The principal regular satellites of gas giants are thought to be formed by the accumulation of solid materials in circumplanetary disks (CPDs). While there has been significant progress in the study of satellite formation in CPDs, details of the supply of satellite building blocks to CPDs remain unclear. We perform the orbital integration of solid particles in the protoplanetary disk (PPD) approaching a planet, considering the gas drag force by using the results of three-dimensional hydrodynamical simulations of a local region around the planet. We investigate the planetary mass dependence of the capture positions and the capture rates of dust particles accreting onto the CPD. We also examine the degree of dust retention in the accreting gas onto the CPD, which is important for determining the ratio of the dust-to-gas inflow rates, a key parameter in satellite formation. We find that the degree of dust retention increases with increasing planetary mass for a given dust scale height in the PPD. In the case of a small planet (M p = 0.2M Jup), most particles with insufficient initial altitudes in the PPD are isolated from the gas in the accreting region. On the other hand, in the case of a massive planet (M p = 1M Jup), dust particles can be coupled to the vertically accreting gas, even when the dust scale height is about 10%-30% of the gas scale height. The results of this study can be used for models of dust delivery and satellite formation in the CPDs of gas giants of various masses, including exoplanets.

DOI： 10.3847/1538-4357/ad4035

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ad4035/pdf

掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We present the results of our mosaic observations of a single Class 0 protostar IRAS 15398−3359 with the Atacama Compact Array in the CO J = 2-1 line. The new observations covering a ∼ 2 ′ square region reveal elongated redshifted and blueshifted components, which are located at distances of ∼30″-75″ on the northern and southern sides of the protostar, respectively, in addition to the previously observed primary and secondary outflows. These elongated components exhibit Hubble-law-like velocity structures, i.e., an increase of velocity with increasing distance from the protostar, suggesting that they comprise the third outflow associated with the protostar. Besides, a new redshifted component is detected at radii of ∼40″-75″ on the northwestern side of the protostar. This redshifted component also exhibits a Hubble-law-like velocity profile, which could be a counterpart of the secondary outflow mostly detected at blueshifted velocities in a previous study. The three outflows are all misaligned by ∼20°-90°, and the dynamical timescale of the primary outflow is shorter than those of the other outflows, approximately by an order of magnitude. These facts hint that the outflow launch direction has significantly changed with time. The outflow direction may change if the rotational axis and the magnetic field are misaligned or if the dense core is turbulent. We favor the second scenario as the origin of the multiple outflows in IRAS 15398−3359, based on a comparison between the observational results and numerical simulations.

DOI： 10.3847/1538-4357/ad34b7

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations have revealed an increasing number of compact protostellar disks with radii of less than a few tens of astronomical units and that young Class 0/I objects have an intrinsic size diversity. To deepen our understanding of the origin of such tiny disks, we have performed highest-resolution configuration observations with ALMA at a beam size of ∼0.″03 (4 au) on the very low-luminosity Class 0 protostar embedded in the Taurus dense core MC 27/L1521F. The 1.3 mm continuum measurement successfully resolved a tiny, faint (∼1 mJy) disk with a major axis length of ∼10 au, one of the smallest examples in the ALMA protostellar studies. In addition, we detected spike-like components in the northeastern direction at the disk edge. Gravitational instability or other fragmentation mechanisms cannot explain the structures, given the central stellar mass of ∼0.2 M ⊙ and the disk mass of ≳10−4 M ⊙. Instead, we propose that these small spike structures were formed by a recent dynamic magnetic flux transport event due to interchange instability that would be favorable to occur if the parental core has a strong magnetic field. The presence of complex arc-like structures on a larger (∼2000 au) scale in the same direction as the spike structures suggests that the event was not single. Such episodic, dynamical events may play an important role in maintaining the compact nature of the protostellar disk in the complex gas envelope during the main accretion phase.

DOI： 10.3847/1538-4357/ad2f9a

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Outflows play a pivotal role in star formation as one of its most visible markers and a means of transporting mass, momentum, and angular momentum from the infalling gas into the surrounding molecular cloud. Their wide reach (at least thousands of astrnomical units) is a contrast to typical disk sizes (∼10–100 au). We employ high-resolution three-dimensional nested-grid nonideal magnetohydrodynamic (MHD) simulations to study outflow properties in the Class 0 phase. We find that no disk wind is driven from the extended centrifugal disk that has weak magnetic coupling. The low-velocity winds emerge instead from the infalling magnetic pseudodisk. Much of the disk actually experiences an infall of matter rather than outflowing gas. Some of the pseudodisk wind (PD-wind) moves inward to regions above the disk and either falls onto the disk or proceeds upward. The upward flow gives the impression of a disk wind above a certain height even if the gas is originally emerging from the pseudodisk. The PD-wind has the strongest flow coming from a disk interaction zone that lies just outside the disk and is an interface between the inwardly advected magnetic field of the pseudodisk and the outwardly diffusing magnetic field of the disk. The low-velocity wind exhibits the features of a flow driven by the magnetic pressure gradient force in some regions and those of a magnetocentrifugal wind in other regions. We interpret the structure and dynamics of the outflow zone in terms of the basic physics of gravity, angular momentum, magnetic fields, and nonideal MHD.

DOI： 10.3847/1538-4357/ad1bf3

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出版者・発行元：Monthly Notices of the Royal Astronomical Society  

Spirals and streamers are the hallmarks of mass accretion during the early stages of star formation. We present the first observations of a large-scale spiral and a streamer towards a very young brown dwarf candidate in its early formation stages. These observations show, for the first time, the influence of external environment that results in asymmetric mass accretion via feeding filaments on to a candidate proto-brown dwarf in the making. The impact of the streamer has produced emission in warm carbon-chain species close to the candidate proto-brown dwarf. Two contrasting scenarios, a pseudo-disc twisted by core rotation and the collision of dense cores, can both explain these structures. The former argues for the presence of a strong magnetic field in brown dwarf formation while the latter suggests that a minimal magnetic field allows large-scale spirals and clumps to form far from the candidate proto-brown dwarf.

DOI： 10.1093/mnras/stae724

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We present molecular line observations of the protostellar outflow associated with HH270mms1 in the Orion B molecular cloud with ALMA. The 12CO (J = 3−2) emissions show that the outflow velocity structure consists of four distinct components of low (≲10 km s−1), intermediate (∼10-25 km s−1) and high (≳40 km s−1) velocities in addition to the entrained gas velocity (∼25-40 km s−1). The high- and intermediate-velocity flows have well-collimated structures surrounded by the low-velocity flow. The chain of knots is embedded in the high-velocity flow or jet, which is the evidence of episodic mass ejections induced by time-variable mass accretion. We could detect the velocity gradients perpendicular to the outflow axis in both the low- and intermediate-velocity flows. We confirmed the rotation of the envelope and disk in the 13CO and C17O emission and found that their velocity gradients are the same as those of the outflow. Thus, we concluded that the velocity gradients in the low- and intermediate-velocity flows are due to the outflow rotation. Using observational outflow properties, we estimated the outflow launching radii to be 67.1-77.1 au for the low-velocity flow and 13.3-20.8 au for the intermediate-velocity flow. Although we could not detect the rotation in the jets due to the limited spatial resolution, we estimated the jet launching radii to be (2.36-3.14) × 10−2 au using the observed velocity of each knot. Thus, the jet is driven from the inner disk region. We could identify the launching radii of distinct velocity components within a single outflow with all the prototypical characteristics expected from recent theoretical works.

DOI： 10.3847/1538-4357/ad19ce

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We present ∼0.″2 (∼80 au) resolution observations of the CO(2-1) and SiO(5-4) lines made with the Atacama large millimeter/submillimeter array toward an extremely young intermediate-mass protostellar source (t dyn < 1000 yr), MMS 1 located in the Orion Molecular Cloud-3 region. We have successfully imaged a very compact CO molecular outflow associated with MMS 1, having deprojected lobe sizes of ∼1800 au (redshifted lobe) and ∼2800 au (blueshifted lobe). We have also detected an extremely compact (≲1000 au) and collimated SiO protostellar jet within the CO outflow. The maximum deprojected jet speed is measured to be as high as 93 km s−1. The SiO jet wiggles and displays a chain of knots. Our detection of the molecular outflow and jet is the first direct evidence that MMS 1 already hosts a protostar. The position-velocity diagram obtained from the SiO emission shows two distinct structures: (i) bow shocks associated with the tips of the outflow, and (ii) a collimated jet, showing the jet velocities linearly increasing with the distance from the driving source. Comparisons between the observations and numerical simulations quantitatively share similarities such as multiple-mass ejection events within the jet and Hubble-like flow associated with each mass ejection event. Finally, while there is a weak flux decline seen in the 850 μm light curve obtained with the James Clerk Maxwell Telescope/SCUBA 2 toward MMS 1, no dramatic flux change events are detected. This suggests that there has not been a clear burst event within the last 8 yr.

DOI： 10.3847/1538-4357/ad2268

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

We present the Atacama Large Millimeter/submillimeter Array observations of linearly polarized 1.1 mm continuum emission at ∼0.″14 (55 au) resolution and CO (J = 2−1) emission at ∼1.″5 (590 au) resolution toward one prestellar (MMS 4), four Class 0 (MMS 1, MMS 3, MMS 5, and MMS 6), one Class I (MMS 7), and one flat-spectrum (MMS 2) sources in the Orion Molecular Cloud 3 region. The dust disk-like structures and clear CO outflows are detected toward all sources except for MMS 4. The diameters of these disk-like structures, ranging from 16 to 97 au, are estimated based on the deconvolved full width half maximum (FWHM) values obtained from the multi-Gaussian fitting. Polarized emissions are detected toward MMS 2, MMS 5, MMS 6, and MMS 7, while no polarized emission is detected toward MMS 1, MMS 3, and MMS 4. MMS 2, MMS 5, and MMS 7 show organized polarization vectors aligned with the minor axes of the disk-like structures, with mean polarization fractions ranging from 0.6% to 1.2%. The strongest millimeter source, MMS 6, exhibits complex polarization orientations and a remarkably high polarization fraction of ∼10% around the Stokes I peak, and 15%-20% on the arm-like structure, as reported by Takahashi et al. (2019). The origins of the polarized emission, such as self-scattering and dust alignment due to the magnetic field or radiative torque, are discussed for individual sources. Some disk-like sources exhibit a polarized intensity peak shift toward the nearside of the disk, which supports that the polarized emission originates from self-scattering.

DOI： 10.3847/1538-4357/ad182d

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

WL 17 is a Class I object and was considered to have a ring–hole structure. We analyzed the structure around WL 17 to investigate the detailed properties of this object. We used Atacama Large Millimeter/submillimeter Array archival data, which have a higher angular resolution than previous observations. We investigated the WL 17 system with the 1.3 mm dust continuum and 12CO and C18O (J = 2–1) line emissions. The dust continuum emission showed a clear ring structure with inner and outer edges of ∼11 and ∼21 au, respectively. In addition, we detected an inner disk of <5 au radius enclosing the central star within the ring, the first observation of this structure. Thus, WL 17 has a ring–gap structure, not a ring–hole structure. We did not detect any marked emission in either the gap or inner disk, indicating that there is no sign of a planet, circumplanetary disk, or binary companion. We identified the source of both blueshifted and redshifted outflows based on the 12CO emission, which is clearly associated with the disk around WL 17. The outflow mass ejection rate is ∼3.6 × 10‑7 M ⊙ yr‑1 and the dynamical timescale is as short as ∼104 yr. The C18O emission showed that an inhomogeneous infalling envelope, which can induce episodic mass accretion, is distributed in the region within ∼1000 au from the central protostar. With these new findings, we can constrain the scenarios of planet formation and dust growth in the accretion phase of star formation.

DOI： 10.3847/1538-4357/ad12b5

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DOI： 10.1016/j.jnucmat.2023.154786

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DOI： 10.1051/0004-6361/202244692

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DOI： 10.1051/0004-6361/202244690

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

We present the detection of a secondary outflow associated with a Class I source, Ser-emb 15, in the Serpens Molecular Cloud. We reveal two pairs of molecular outflows consisting of three lobes, that is, primary and secondary outflows, using Atacama Large Millimeter/submillimeter Array ¹²CO and SiO line observations at a resolution of ∼318 au. The secondary outflow is elongated approximately perpendicular to the axis of the primary outflow in the plane of the sky. We also identify two compact structures, Sources A and B, within an extended structure associated with Ser-emb 15 in the 1.3 mm continuum emission at a resolution of ∼40 au. The projected sizes of Sources A and B are 137 au and 60 au, respectively. Assuming a dust temperature of 20 K, we estimate the dust mass to be 2.4 × 10⁻³M_⊙ for Source A and 3.3 × 10⁻⁴M_⊙ for Source B. C¹⁸O line data imply rotational motion around the extended structure, but we cannot resolve rotational motion in Source A and/or B because the angular and frequency resolutions are insufficient. Therefore, we cannot conclude whether Ser-emb 15 is a single or binary system. Thus, either Source A or Source B could drive the secondary outflow. We discuss two scenarios that might explain the driving mechanism of the primary and secondary outflows: the Ser-emb 15 system is (1) a binary system composed of Sources A and B, or (2) a single-star system composed of Source A alone. In either case, the system could be a suitable target for investigating the disk and/or binary formation processes in complicated environments. Detecting these outflows should contribute to understanding complex star-forming environments, which may be common in the star formation processes.

DOI： 10.3847/1538-4357/ad0132

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ad0132/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal Letters  

Transferring a significant fraction of the magnetic flux from a dense cloud core is essential in the star formation process. A ringlike structure produced by magnetic flux loss has been predicted theoretically, but no observational identification has been presented. We have performed ALMA observations of the Class I protostar IRS 2 in the Corona Australis star-forming region and resolved a distinctive gas ring in the C18O (J = 2-1) line emission. The center of this gas ring is ∼5000 au away from the protostar, with a diameter of ∼7000 au. The radial velocity of the gas is ≲ 1 km s−1 blueshifted from that of the protostar, with a possible expanding feature judged from the velocity-field (moment 1) map and position-velocity diagram. These features are either observationally new or have been discovered but not discussed in depth because they are difficult to explain by well-studied protostellar phenomena such as molecular outflows and accretion streamers. A plausible interpretation is a magnetic wall created by the advection of magnetic flux, which is theoretically expected in the Class 0/I phase during star formation as a removal mechanism of magnetic flux. Similar structures reported in the other young stellar sources could likely be candidates formed by the same mechanism, encouraging us to revisit the issue of magnetic flux transport in the early stages of star formation from an observational perspective.

DOI： 10.3847/2041-8213/acfca9

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出版者・発行元：Progress of Theoretical and Experimental Physics  

The detections of gravitational waves (GW) by the LIGO/Virgo collaborations provide various possibilities for both physics and astronomy. We are quite sure that GW observations will develop a lot, both in precision and in number, thanks to the continuous work on the improvement of detectors, including the expected new detector, KAGRA, and the planned detector, LIGO-India. On this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development, focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics.

DOI： 10.1093/ptep/ptab042

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

Recent millimeter/submillimeter facilities have revealed the physical properties of filamentary molecular clouds in relation to high-mass star formation. A uniform survey of the nearest, face-on star-forming galaxy, the Large Magellanic Cloud (LMC), complements the Galactic knowledge. We present ALMA survey data with a spatial resolution of ∼0.1 pc in the 0.87 mm continuum and HCO⁺ (4–3) emission toward 30 protostellar objects with luminosities of 10⁴–10^5.5L_⊙ in the LMC. The spatial distributions of the HCO⁺ (4–3) line and thermal dust emission are well correlated, indicating that the line effectively traces dense, filamentary gas with an H₂ volume density of ≳10⁵ cm⁻³ and a line mass of ∼10³–10⁴M_⊙ pc⁻¹. Furthermore, we obtain an increase in the velocity line widths of filamentary clouds, which follows a power-law dependence on their H₂ column densities with an exponent of ∼0.5. This trend is consistent with observations toward filamentary clouds in nearby star-forming regions within ≲1 kpc from us and suggests enhanced internal turbulence within the filaments due to surrounding gas accretion. Among the 30 sources, we find that 14 are associated with hub-filamentary structures, and these complex structures predominantly appear in protostellar luminosities exceeding ∼5 × 10⁴L_⊙. The hub-filament systems tend to appear in the latest stages of their natal cloud evolution, often linked to prominent H ii regions and numerous stellar clusters. Our preliminary statistics suggest that the massive filaments accompanied by hub-type complex features may be a necessary intermediate product in forming extremely luminous high-mass stellar systems capable of ultimately dispersing the parent cloud.

DOI： 10.3847/1538-4357/acefb7

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/acefb7/pdf

DOI： 10.3847/2041-8213/acdb60

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Publications of the Astronomical Society of Japan  

We propose a new evolutionary process for protoplanetary disks, the co-evolution of dust grains and protoplanetary disks, revealed by dust-gas two-fluid non-ideal magnetohydrodynamics simulations considering the growth of dust grains and associated changes in magnetic resistivity. We found that the dust growth significantly affects disk evolution by changing the coupling between the gas and the magnetic field. Moreover, once the dust grains grow sufficiently large and the adsorption of charged particles on to them becomes negligible, the physical quantities (e.g., density and magnetic field) of the disk are well described by characteristic power laws. In this disk structure, the radial profile of density is steeper and the disk mass is smaller than those of the model ignoring dust growth. We analytically derive these power laws from the basic equations of non-ideal magnetohydrodynamics. The analytical power laws are determined only by observable physical quantities, e.g., central stellar mass and mass accretion rate, and do not include difficult-to-determine parameters, e.g., the viscous parameter & alpha;. Therefore, our model is observationally testable and this disk structure is expected to provide a new perspective for future studies on protostar and disk evolution.

DOI： 10.1093/pasj/psad040

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

Intermediate-mass black holes (with ≥10⁵M_⊙) are promising candidates for the origin of supermassive black holes (with ∼10⁹M_⊙) in the early universe (redshift z ∼ 6). Chon & Omukai first pointed out direct collapse black hole (DCBH) formation in metal-enriched atomic-cooling halos (ACHs), which relaxes the DCBH formation criterion. On the other hand, Hirano et al. showed that magnetic effects promote DCBH formation in metal-free ACHs. We perform a set of magnetohydrodynamical simulations to investigate star formation in magnetized ACHs with metallicities Z/Z_⊙ = 0, 10⁻⁵, and 10⁻⁴. Our simulations show that the mass accretion rate onto the protostars becomes lower in metal-enriched ACHs than in metal-free ACHs. However, many protostars form from gravitationally and thermally unstable metal-enriched gas clouds. Under such circumstances, the magnetic field rapidly increases as magnetic field lines wind up due to the spin of protostars. The region with the amplified magnetic field expands outwards due to the orbital motion of protostars and the rotation of the accreting gas. The amplified magnetic field extracts angular momentum from the accreting gas, promotes the coalescence of low-mass protostars, and increases the mass growth rate of the primary protostar. We conclude that the magnetic field amplification is always realized in metal-enriched ACHs regardless of the initial magnetic field strength, which affects the DCBH formation criterion. In addition, we find a qualitatively different trend from the previous unmagnetized simulations in that the mass growth rate is maximal for extremely metal-poor ACHs with Z/Z_⊙ = 10⁻⁵.

DOI： 10.3847/1538-4357/acda94

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/acda94/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Monthly Notices of the Royal Astronomical Society  

We investigate the influence of dust particle size evolution on non-ideal magnetohydrodynamic (MHD) effects during the collapsing phase of star-forming cores, taking both the turbulence intensity in the collapsing cloud core and the fragmentation velocity of dust particles as parameters. When the turbulence intensity is small, the dust particles do not grow significantly, and the non-ideal MHD effects work efficiently in high-density regions. The dust particles rapidly grow in a strongly turbulent environment, while the efficiency of non-ideal MHD effects in such an environment depends on the fragmentation velocity of the dust particles. When the fragmentation velocity is small, turbulence promotes coagulation growth and collisional fragmentation of dust particles, producing small dust particles. In this case, the adsorption of charged particles on the dust particle surfaces becomes efficient and the abundance of charged particles decreases, making non-ideal MHD effects effective at high densities. On the other hand, when the fragmentation velocity is high, dust particles are less likely to fragment, even if the turbulence is strong. In this case, the production of small dust particles becomes inefficient and non-ideal MHD effects become less effective. We also investigate the effect of the dust composition on the star and disc formation processes. We constrain the turbulence intensity of a collapsing core and the fragmentation velocity of dust for circumstellar disc formation due to the dissipation of the magnetic field.

DOI： 10.1093/mnras/stad1241

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Monthly Notices of the Royal Astronomical Society  

We present results from the first molecular line survey to search for the fundamental complex organic molecule, methanol (CH3OH), in 14 Class 0/I proto-brown dwarfs (proto-BDs). IRAM 30-m observations over the frequency range of 92-116 and 213-280 GHz have revealed emission in 14 CH3OH transition lines, at upper state energy level, Eupper ∼7-49 K, and critical densities, ncrit of 105-109 cm−3. The most commonly detected lines are at Eupper < 20 K, while 11 proto-BDs also show emission in the higher excitation lines at Eupper ∼21-49 K and ncrit ∼ 105 to 108 cm−3. In comparison with the brown dwarf formation models, the high excitation lines likely probe the warm (∼25-50 K) corino region at ∼10-50 au in the proto-BDs, while the low-excitation lines trace the cold (<20 K) gas at ∼50-150 au. The column density for the cold component is an order of magnitude higher than the warm component. The CH3OH ortho-to-para ratios range between ∼0.3 and 2.3. The volume-averaged CH3OH column densities show a rise with decreasing bolometric luminosity among the proto-BDs, with the median column density higher by a factor of ∼3 compared to low-mass protostars. Emission in high-excitation (Eupper > 25 K) CH3OH lines together with the model predictions suggest that a warm corino is present in ∼78 per cent of the proto-BDs in our sample. The remaining shows evidence of only the cold component, possibly due to the absence of a strong, high-velocity jet that can stir up the warm gas around it.

DOI： 10.1093/mnras/stad1329

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astronomical Journal  

The Subaru telescope is currently performing a strategic program (SSP) using the high-precision near-infrared (NIR) spectrometer IRD to search for exoplanets around nearby mid/late M dwarfs via radial velocity (RV) monitoring. As part of the observing strategy for the exoplanet survey, signatures of massive companions such as RV trends are used to reduce the priority of those stars. However, this RV information remains useful for studying the stellar multiplicity of nearby M dwarfs. To search for companions around such "deprioritized" M dwarfs, we observed 14 IRD-SSP targets using Keck/NIRC2 with pyramid wave-front sensing at NIR wavelengths, leading to high sensitivity to substellar-mass companions within a few arcseconds. We detected two new companions (LSPM J1002+1459 B and LSPM J2204+1505 B) and two new candidates that are likely companions (LSPM J0825+6902 B and LSPM J1645+0444 B), as well as one known companion. Including two known companions resolved by the IRD fiber injection module camera, we detected seven (four new) companions at projected separations between similar to 2 and 20 au in total. A comparison of the colors with the spectral library suggests that LSPM J2204+1505 B and LSPM J0825+6902 B are located at the boundary between late M and early L spectral types. Our deep high-contrast imaging for targets where no bright companions were resolved did not reveal any additional companion candidates. The NIRC2 detection limits could constrain potential substellar-mass companions (similar to 10-75 M (Jup)) at 10 au or further. The failure with Keck/NIRC2 around the IRD-SSP stars having significant RV trends makes these objects promising targets for further RV monitoring or deeper imaging with the James Webb Space Telescope to search for smaller-mass companions below the NIRC2 detection limits.

DOI： 10.3847/1538-3881/acbf37

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

We report on an Atacama Large Millimeter/submillimeter Array study of the Class I or II intermediate-mass protostar DK Cha in the Chamaeleon II region. The ¹²CO(J = 2–1) images have an angular resolution of ∼1″ (∼250 au) and show high-velocity blueshifted (≳70 km s⁻¹) and redshifted (≳50 km s⁻¹) emissions, which have 3000 au scale crescent-shaped structures around the protostellar disk traced in the 1.3 mm continuum. Because the high-velocity components of the CO emission are associated with the protostar, we concluded that the emission traces the pole-on outflow. The blueshifted outflow lobe has a clear layered velocity gradient with a higher-velocity component located on the inner side of the crescent shape, which can be explained by a model of an outflow with a higher velocity in the inner radii. Based on the directly driven outflow scenario, we estimated the driving radii from the observed outflow velocities and found that the driving region extends over 2 orders of magnitude. The ¹³CO emission traces a complex envelope structure with arc-like substructures with lengths of ∼1000 au. We identified the arc-like structures as streamers because they appear to be connected to a rotating infalling envelope. DK Cha is useful for understanding characteristics that are visible by looking at nearly face-on configurations of young protostellar systems, providing an alternative perspective for studying the star formation process.

DOI： 10.3847/1538-4357/acb930

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/acb930/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Monthly Notices of the Royal Astronomical Society  

We calculate the evolution of cloud cores embedded in different envelopes to investigate environmental effects on the mass accretion rate on to protostars. As the initial state, we neglect the magnetic field and cloud rotation, and adopt star-forming cores composed of two parts: a centrally condensed core and an outer envelope. The inner core has a critical Bonnor-Ebert density profile and is enclosed by the outer envelope. We prepare 15 star-forming cores with different outer envelope densities and gravitational radii, within which the gas flows into the collapsing core, and calculate their evolution until ∼2 × 105 yr after protostar formation. The mass accretion rate decreases as the core is depleted when the outer envelope density is low. In contrast, the mass accretion rate is temporarily enhanced when the outer envelope density is high and the resultant protostellar mass exceeds the initial mass of the centrally condensed core. Some recent observations indicate that the mass of pre-stellar cores is too small to reproduce the stellar mass distribution. Our simulations show that the mass inflow from outside the core contributes greatly to protostellar mass growth when the core is embedded in a high-density envelope, which could explain the recent observations.

DOI： 10.1093/mnras/stac3819

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DOI： 10.1093/pasj/psac045

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DOI： 10.1093/pasj/psac103

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

We present a high-angular resolution (∼1″) and wide-field ($2uildrel{,prime}over{.} 9 imes 1uildrel{,prime}over{.} 9$) image of the 1.3 mm continuum, CO(J = 2–1) and SiO(J = 5–4) line emissions toward an embedded protocluster, FIR 3, FIR 4, and FIR 5, in the Orion Molecular Cloud 2 obtained from the Atacama Large Millimeter/submillimeter Array. We identify 51 continuum sources, 36 of which are newly identified in this study. Their dust masses, projected sizes, and H₂ gas number densities are estimated to be 3.8 × 10⁻⁵–1.1 × 10⁻²M_⊙, 290–2000 au, and 6.4 × 10⁶–3.3 × 10⁸ cm⁻³, respectively. The results of a Jeans analysis show that ∼80% of the protostellar sources and ∼15% of the prestellar sources are gravitationally bound. We identify 12 molecular outflows traced in the CO(J = 2–1) emission, six of which are newly detected. We spatially resolve shocked gas structures traced by the SiO(J = 5–4) emission in this region for the first time. We identify shocked gas originating from outflows and other shocked regions. These results provide direct evidence of an interaction between dust condensation, FIR 4, and an energetic outflow driven by HOPS-370 located within FIR 3. A comparison of the outflow dynamical timescales, fragmentation timescales, and protostellar ages shows that the previously proposed triggered star formation scenario in FIR 4 is not strongly supported. We also discuss the spatial distribution of filaments identified in our continuum image by comparing it with a previously identified hub-fiber system in the N₂H⁺ line.

DOI： 10.3847/1538-4357/aca7c9

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/aca7c9/pdf

出版者・発行元：Monthly Notices of the Royal Astronomical Society  

We calculate the evolution of a star-forming cloud core using a three-dimensional resistive magnetohydrodynamics simulation, treating dust grains as Lagrangian particles, to investigate the dust motion in the early star formation stage. We prepare six different-sized set of dust particles in the range ad = 0.01-1000 μm, where ad is the dust grain size. In a gravitationally collapsing cloud, a circumstellar disk forms around a protostar and drives a protostellar outflow. Almost all the small dust grains (ad 10-100 μm) initially distributed in the region θ0 45° are ejected from the center by the outflow, where θ0 is the initial zenith angle relative to the rotation axis, whereas only a small number of the large dust grains (a d gtrsim 100 μm) distributed in the region are ejected. All other grains fall onto either the protostar or disk without being ejected by the outflow. Regardless of the dust grain size, the behavior of the dust motion is divided into two trends after dust particles settle into the circumstellar disk. The dust grains reaching the inner disk region from the upper envelope preferentially fall onto the protostar, while those reaching the outer disk region or disk outer edge from the envelope can survive without an inward radial drift. These surviving grains can induce dust growth. Thus, we expect that the outer disk regions could be a favored place of planet formation.

DOI： 10.1093/mnras/stac3503

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Proceedings of the International Astronomical Union  

We report on a direct comparison of VLBI maser data and ALMA thermal-emission data for the high-mass protostar G353.273+0.641. We detected a gravitationally-unstable disk by dust and a high-velocity jet traced by a thermal CO line by ALMA long-baselines (LB). 6.7 GHz CH3OH masers trace infalling streamlines inside the disk. The innermost maser ring indicates another compact accretion disk of 30 au. Such a nested system could be caused by angular momentum transfer by the spiral arms. 22 GHz H2O masers trace the jet-accelerating region, which are directly connecting the CO jet and the protostar. The recurrent maser flares imply episodic jet ejections per 1–2 yr, while typical separation of CO knots indicates a variation of outflow rate per 100 yr. Our study demonstrates that VLBI maser observations are still a powerful tool to explore detailed structures nearby high-mass protostars by combining ALMA LB.

DOI： 10.1017/s1743921323003216

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Publications of the Astronomical Society of Japan  

Understanding the initial conditions of star formation requires both
observational studies and theoretical works taking into account the magnetic
field, which plays an important role in star formation processes. Herein, we
study the young nearby dense cloud core L1521 F ($n$(H$_2$) $sim 10^{4-6}$
cm$^{-3}$) in the Taurus Molecular Cloud. This dense core hosts a 0.2 $M_odot$
protostar, categorized as a Very Low Luminosity Objects with complex velocity
structures, particularly in the vicinity of the protostar. To trace the
magnetic field within the dense core, we conducted high sensitivity
submillimeter polarimetry of the dust continuum at $lambda$= 850 $mu$m and
450 $mu$m using the POL-2 polarimeter situated in front of the SCUBA-2
submillimeter bolometer camera on James Clerk Maxwell Tetescope. This was
compared with millimeter polarimetry taken at $lambda$= 3.3 mm with ALMA. The
magnetic field was detected at $lambda$= 850 $mu$m in the peripheral region,
which is threaded in a north-south direction, while the central region traced
at $lambda$= 450 $mu$m shows a magnetic field with an east-west direction,
i.e., orthogonal to that of the peripheral region. Magnetic field strengths are
estimated to be $sim$70 $mu$G and 200 $mu$G in the peripheral- and
central-regions, respectively, using the Davis-Chandrasekhar-Fermi method. The
resulting mass-to-flux ratio of 3 times larger than that of magnetically
critical state for both regions indicates that L1521 F is magnetically
supercritical, i.e., gravitational forces dominate over magnetic turbulence
forces. Combining observational data with MHD simulations, detailed parameters
of the morphological properties of this puzzling object are derived for the
first time.

DOI： 10.1093/pasj/psac094

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その他リンク： http://arxiv.org/pdf/2211.08988v1

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Monthly Notices of the Royal Astronomical Society  

The aim of this study is to examine dust dynamics on a large scale and investigate the coupling of dust with gas fluid in the star formation process. We propose a method for calculating the dust trajectory in a gravitationally collapsing cloud, where the dust grains are treated as Lagrangian particles and are assumed to be neutral. We perform the dust trajectory calculations in combination with non-ideal magnetohydrodynamics simulation. Our simulation shows that dust particles with a size of $le 10, {
m mu m}$ are coupled with gas in a star-forming cloud core. We investigate the time evolution of the dust-to-gas mass ratio and the Stokes number, which is defined as the stopping time normalized by the freefall time-scale, and show that large dust grains ($gtrsim 100, {
m mu m}$) have a large Stokes number (close to unity) and tend to concentrate in the central region (i.e. protostar and rotationally supported disc) faster than do small grains ($lesssim 10, {
m mu m}$). Thus, large grains significantly increase the dust-to-gas mass ratio around and inside the disc. We also confirm that the dust trajectory calculations, which trace the physical quantities of each dust particle, reproduce previously reported results obtained using the Eulerian approach.

DOI： 10.1093/mnras/stac2115

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal Letters  

Abstract

Protostellar outflows are one of the most outstanding features of star formation. Observational studies over the last several decades have successfully demonstrated that outflows are ubiquitously associated with low- and high-mass protostars in solar-metallicity Galactic conditions. However, the environmental dependence of protostellar outflow properties is still poorly understood, particularly in the low-metallicity regime. Here we report the first detection of a molecular outflow in the Small Magellanic Cloud with 0.2 Z_⊙, using Atacama Large Millimeter/submillimeter Array observations at a spatial resolution of 0.1 pc toward the massive protostar Y246. The bipolar outflow is nicely illustrated by high-velocity wings of CO(3–2) emission at ≳15 km s⁻¹. The evaluated properties of the outflow (momentum, mechanical force, etc.) are consistent with those of the Galactic counterparts. Our results suggest that the molecular outflows, i.e., the guidepost of the disk accretion at the small scale, might be universally associated with protostars across the metallicity range of ∼0.2–1 Z_⊙.

DOI： 10.3847/2041-8213/ac81c1

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その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/ac81c1/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

The canonical theory of star formation in a magnetized environment predicts the formation of hourglass-shaped magnetic fields during the prestellar collapse phase. In protostellar cores, recent observations reveal complex and strongly distorted magnetic fields in the inner regions that are sculpted by rotation and outflows. We conduct resistive, nonideal magnetohydrodynamic simulations of a protostellar core and employ the radiative transfer code POLARIS to produce synthetic polarization segment maps. A comparison of our mock-polarization maps based on the toroidal-dominated magnetic field in the outflow zone with the observed polarization vectors of SiO lines in Orion Source I shows a reasonable agreement when the magnetic axis is tilted at an angle θ = 15° with respect to the plane of the sky and if the SiO lines have a net polarization parallel to the local magnetic field. Although the observed polarization is from SiO lines and our synthetic maps are due to polarized dust emission, a comparison is useful and allows us to resolve the ambiguity of whether the line polarization is parallel or perpendicular to the local magnetic field direction.

DOI： 10.3847/1538-4357/ac7c0f

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Monthly Notices of the Royal Astronomical Society  

We determine the time-evolution of the dust particle size distribution during the collapse of a cloud core, accounting for both dust coagulation and dust fragmentation, to investigate the influence of dust growth on non-ideal magnetohydrodynamic (MHD) effects. The density evolution of the collapsing core is given by a one-zone model. We assume two types of dust model: dust composed only of silicate (silicate dust) and dust with a surface covered by H2O ice (H2O ice dust). When only considering collisional coagulation, the non-ideal MHD effects are not effective in the high-density region for both the silicate and H2O ice dust cases. This is because dust coagulation reduces the abundance of small dust particles, resulting in less efficient adsorption of charged particles on the dust surface. For the silicate dust case, when collisional fragmentation is included, the non-ideal MHD effects do apply at a high density of nH > 1012 cm-3 because of the abundant production of small dust particles. On the other hand, for the H2O ice dust case, the production of small dust particles due to fragmentation is not efficient. Therefore, for the H2O ice dust case, non-ideal magnetohydrodynamic effects apply only in the range nH ≳ 1014 cm-3, even when collisional fragmentation is considered. Our results suggest that it is necessary to consider both dust collisional coagulation and fragmentation to activate non-ideal magnetohydrodynamic effects, which should play a significant role in the star and disc formation processes.

DOI： 10.1093/mnras/stac1919

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記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal  

Abstract

Gas accretion onto the circumplanetary disks and the source region of accreting gas are important to reveal dust accretion that leads to satellite formation around giant planets. We performed local three-dimensional high-resolution hydrodynamic simulations of an isothermal and inviscid gas flow around a planet to investigate the planetary-mass dependence of the gas accretion bandwidth and gas accretion rate onto circumplanetary disks. We examined cases with various planetary masses corresponding to M_p = 0.05–1M_Jup at 5.2 au, where M_Jup is the current Jovian mass. We found that the radial width of the gas accretion band is proportional to ${M}_{ {
m{p } } }^{1/6}$ for the low-mass regime with M_p ≲ 0.2M_Jup while it is proportional to M_p for the high-mass regime with M_p ≳ 0.2M_Jup. We found that the ratio of the mass accretion rate onto the circumplanetary disk to that into the Hill sphere is about 0.4 regardless of the planetary mass for the cases we examined. Combining our results with the gap model obtained from global hydrodynamic simulations, we derive a semi-analytical formula of mass accretion rate onto circumplanetary disks. We found that the mass dependence of our three-dimensional accretion rates is the same as the previously obtained two-dimensional case, although the qualitative behavior of accretion flow onto the circumplanetary disk is quite different between the two cases.

DOI： 10.3847/1538-4357/ac7ddf

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その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ac7ddf/pdf

記述言語：その他   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astrophysical Journal Letters  

Abstract One critical remaining issue that is unclear in the initial mass function of the first (Population III) stars is the final fate of secondary protostars that formed in the accretion disk—specifically, whether they merge or survive. We focus on the magnetic effects on the formation of the first star under a cosmological magnetic field. We perform a suite of ideal magnetohydrodynamic simulations for 1000 yr after the first protostar formation. Instead of the sink particle technique, we employ a stiff equation of state approach to represent the magnetic field structure connecting protostars. Ten years after the first protostar formation in the cloud initialized with B₀ = 10⁻²⁰ G at n₀ = 10⁴ cm⁻³, the magnetic field strength around the protostars has amplified from pico- to kilo-Gauss, which is the same strength as the present-day star. The magnetic field rapidly winds up since the gas in the vicinity of the protostar (≤10 au) has undergone several tens of orbital rotations in the first decade after protostar formation. As the mass accretion progresses, the vital magnetic field region extends outward, and magnetic braking eliminates the fragmentation of the disk that would happen in an unmagnetized model. On the other hand, assuming a gas cloud with a small angular momentum, this amplification might not work because the rotation would be slower. However, disk fragmentation would not occur in that case. We conclude that the exponential amplification of the cosmological magnetic field strength, about 10⁻¹⁸ G, eliminates disk fragmentation around Population III protostars.

DOI： 10.3847/2041-8213/ac85e0

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その他リンク： https://iopscience.iop.org/article/10.3847/2041-8213/ac85e0/pdf

DOI： 10.1021/acs.jpcc.1c08739

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Astronomical Journal  

Detailed chemical analyses of M dwarfs are scarce but necessary to constrain the formation environment and internal structure of planets being found around them. We present elemental abundances of 13 M dwarfs (2900 < T (eff) < 3500 K) observed in the Subaru/IRD planet search project. They are mid- to late-M dwarfs whose abundance of individual elements has not been well studied. We use the high-resolution (similar to 70,000) near-infrared (970-1750 nm) spectra to measure the abundances of Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, and Sr by the line-by-line analysis based on model atmospheres, with typical errors ranging from 0.2 dex for [Fe/H] to 0.3-0.4 dex for other [X/H]. We measure radial velocities from the spectra and combine them with Gaia astrometry to calculate the Galactocentric space velocities UVW. The resulting [Fe/H] values agree with previous estimates based on medium-resolution K-band spectroscopy, showing a wide distribution of metallicity (-0.6 < [Fe/H] < +0.4). The abundance ratios of individual elements [X/Fe] are generally aligned with the solar values in all targets. While the [X/Fe] distributions are comparable to those of nearby FGK stars, most of which belong to the thin-disk population, the most metal-poor object, GJ 699, could be a thick-disk star. The UVW velocities also support this. The results raise the prospect that near-infrared spectra of M dwarfs obtained in the planet search projects can be used to grasp the trend of elemental abundances and the Galactic stellar population of nearby M dwarfs.

DOI： 10.3847/1538-3881/ac3ee0

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出版者・発行元：arXiv  

We present our current understanding of the formation and early evolution of protostars, protoplanetary disks, and the driving of outflows as dictated by the interplay of magnetic fields and partially ionized gas in molecular cloud cores. In recent years, the field has witnessed enormous development through sub-millimeter observations which in turn have constrained models of protostar formation. As a result of these observations % that the observations provided, the state-of-the-art theoretical understanding of the formation and evolution of young stellar objects is described. In particular, we emphasize the importance of the coupling, decoupling, and re-coupling between weakly ionized gas and the magnetic field on appropriate scales. This highlights the complex and intimate relationship between gravitational collapse and magnetic fields in young protostars.

DOI： 10.48550/ARXIV.2209.13765

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記述言語：その他   掲載種別：研究論文（学術雑誌）  

Dust growth and its associated dynamics play key roles in the first phase of planet formation in young stellar objects (YSOs). Observations have detected signs of dust growth in very young protoplanetary disks. Furthermore, signs of planet formation, gaps in the disk at a distance of several 10 astronomical units (AU) from the central protostar are also reported. From a theoretical point of view, however, it is not clear how planet form at the outer region of a disk despite the difficulty due to rapid inward drift of dust so called radial drift barrier. Here, on the basis of three-dimensional magneto-hydrodynamical simulations of disk evolution with the dust growth, we propose a mechanism named "ash-fall" phenomenon induced by powerful molecular outflow driven by magnetic field which may circumvent the radial drift barrier. We found that the large dust which grows to a size of $sim cm$ in the inner region of a disk is entrained by an outflow from the disk. Then large dust decoupled from gas is ejected from the outflow due to centrifugal force, enriching the grown dust in the envelope and is eventually fall onto the outer edge of the disk. The overall process is similar to behaviour of ash-fall from volcanic eruptions. In the ash-fall phenomenon, the Stokes number of dust increases by reaccreting to the less dense disk outer edge. This may make the dust grains overcome the radial drift barrier. Consequently, the ash-fall phenomenon can provide a crucial assist for making the formation of the planetesimals in outer region of the disk possible, and hence the formation of wide-orbit planets and the formation of the gaps....

DOI： 10.3847/2041-8213/ac2b2f

記述言語：その他  

Combining numerical simulations and analytical modeling, we investigate whether close binary systems form by the effect of magnetic braking. Using magnetohydrodynamics simulations, we calculate the cloud evolution with a sink, for which we do not resolve the binary system or binary orbital motion to realize long-term time integration. Then, we analytically estimate the binary separation using the accreted mass and angular momentum obtained from the simulation. In unmagnetized clouds, wide binary systems with separations of >100 au form, in which the binary separation continues to increase during the main accretion phase. In contrast, close binary systems with separations of <100 au can form in magnetized clouds. Since the efficiency of magnetic braking strongly depends on both the strength and configuration of the magnetic field, they also affect the formation conditions of a close binary. In addition, the protostellar outflow has a negative impact on close binary formation, especially when the rotation axis of the prestellar cloud is aligned with the global magnetic field. The outflow interrupts the accretion of gas with small angular momentum, which is expelled from the cloud, while gas with large angular momentum preferentially falls from the side of the outflow onto the binary system and widens the binary separation. This study shows that a cloud with a magnetic field that is not parallel to the rotation axis is a favorable environment for the formation of close binary systems....

記述言語：その他   掲載種別：研究論文（学術雑誌）  

Using three-dimensional magnetohydrodynamics simulations, the driving of protostellar jets is investigated in different star-forming cores with the parameters of magnetic field strength and mass accretion rate. Powerful high-velocity jets appear in strongly magnetized clouds when the mass accretion rate onto the protostellar system is lower than $dot{M} lesssim 10^{-3}, {
m M}_odot$ yr^-1. On the other hand, even at this mass accretion rate range, no jets appear for magnetic fields of prestellar clouds as weak as μ₀ ≳ 5-10, where μ₀ is the mass-to-flux ratio normalized by the critical value (2πG^1/2)^-1. For $dot{M}gtrsim 10^{-3}, {
m M}_odot$ yr^-1, although jets usually appear just after protostar formation independent of the magnetic field strength, they soon weaken and finally disappear. Thus, they cannot help drive the low-velocity outflow when there is no low-velocity flow just before protostar formation. As a result, no significant mass ejection occurs during the early mass accretion phase either when the prestellar cloud is weaky magnetized or when the mass accretion rate is very high. Thus, protostars formed in such environments would trace different evolutionary paths from the normal star formation process....

DOI： 10.1093/mnras/stab2626

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/abfdbb

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/ac0913

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We present Atacama Large Millimeter/submillimeter Array CO (J = 2-1) and 1.3 mm continuum observations of the high-velocity jet associated with the FIR 6b protostar located in the Orion Molecular Cloud-2. We detect a velocity gradient along the short axis of the jet in both the red- and blueshifted components. The position-velocity diagrams along the short axis of the redshifted jet show a typical characteristic of a rotating cylinder. We attribute the velocity gradient in the redshifted component to rotation of the jet. The rotation velocity (>20 km s-1) and specific angular momentum (>1022 cm2 s-1) of the jet around FIR 6b are the largest among all jets in which rotation has been observed. By combining disk wind theory with our observations, the jet launching radius is estimated to be in the range of 2.18-2.96 au. The rapid rotation, large specific angular momentum, and a launching radius far from the central protostar can be explained by a magnetohydrodynamic disk wind that contributes to the angular momentum transfer in the late stages of protostellar accretion.

DOI： 10.3847/1538-4357/ac069f

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/abf5db

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/abb810

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using the Atacama Large Millimeter/submillimeter Array (ALMA), we investigated the peculiar millimeter source MMS 3 located in the Orion Molecular Cloud 3 (OMC-3) region in the 1.3 mm continuum, CO (J = 2-1), SiO (J = 5-4), (CO)-O-18 (J = 2-1), N2D+ (J = 3-2), and DCN (J = 3-2) emissions. With the ALMA high angular resolution (similar to 02), we detected a very compact and highly centrally condensed continuum emission with a size of 045 x 032 (P.A. = 022). The peak position coincides with the locations of previously reported Spitzer/IRAC and X-ray sources within their positional uncertainties. We also detected an envelope with a diameter of similar to 6800 au (P.A. = 75 degrees) in the (CO)-O-18 (J = 2-1) emission. Moreover, a bipolar outflow was detected in the CO (J = 2-1) emission for the first time. The outflow is elongated roughly perpendicular to the long axis of the envelope detected in the (CO)-O-18 (J = 2-1) emission. Compact high-velocity CO gas in the (redshifted) velocity range of 22-30 km s(-1), presumably tracing a jet, was detected near the 1.3 mm continuum peak. A compact and faint redshifted SiO emission was marginally detected in the CO outflow lobe. The physical quantities of the outflow in MMS 3 are somewhat smaller than those in other sources in the OMC-3 region. The centrally condensed object associated with the near-infrared and X-ray sources, the flattened envelope, and the faint outflow indicate that MMS 3 harbors a low-mass protostar with an age of similar to 10(3) yr.

DOI： 10.3847/1538-4357/abe61c

記述言語：その他  

The detections of gravitational waves (GW) by LIGO/Virgo collaborations provide various possibilities to physics and astronomy. We are quite sure that GW observations will develop a lot both in precision and in number owing to the continuous works for the improvement of detectors, including the expectation to the newly joined detector, KAGRA, and the planned detector, LIGO-India. In this occasion, we review the fundamental outcomes and prospects of gravitational wave physics and astronomy. We survey the development focusing on representative sources of gravitational waves: binary black holes, binary neutron stars, and supernovae. We also summarize the role of gravitational wave observations as a probe of new physics....

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/abc6fc

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/abbc08

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/2041-8213/ab9ca8

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have performed survey-type observations in 1 mm continuum and molecular lines toward dense cores (32 prestellar + 7 protostellar) with an average density of greater than or similar to 10(5)cm(-3)in the Taurus molecular clouds using the Atacama Large Millimeter/submillimeter Array-Atacama Compact Array (ALMA-ACA) stand-alone mode with an angular resolution of 65 (similar to 900 au). The primary purpose of this study is to investigate the innermost part of dense cores with view to understanding the initial condition of star formation. In the protostellar cores, contributions from protostellar disks dominate the observed continuum flux with a range of 35%-90%, except for the very low-luminosity object. For the prestellar cores, we have successfully confirmed continuum emission from dense gas with a density of greater than or similar to 3 x 10(5)cm(-3)toward approximately one-third of the targets. Thanks to the lower spatial frequency coverage with the ACA 7 m array, the detection rate is significantly higher than that of the previous surveys, which have zero or one continuum-detected sources among a large number of starless samples using the ALMA Main Array. The statistical counting method tells us that the lifetime of prestellar cores until protostar formation therein approaches the freefall time as the density increases. Among the prestellar cores, at least two targets have possible internal substructures, which are detected in continuum emission with the size scale of similar to 1000 au if we consider the molecular line ((CO)-O-18 and N2D+) distributions. These results suggest that small-scale fragmentation/coalescence processes occur in a region smaller than 0.1 pc, which may determine the final core mass associated with individual protostar formation before starting the dynamical collapse of the core with a central density of similar to(0.3-1) x 10(6)cm(-3).

DOI： 10.3847/1538-4357/ab9ca7

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/ab9f9d

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation of a close binary system is investigated using a three-dimensional resistive magnetohydrodynamic simulation. Starting from a prestellar cloud, the cloud evolution is calculated until similar to 400 yr after protostar formation. Fragmentation occurs in the gravitationally collapsing cloud, and two fragments evolve into protostars. The protostars orbit each other and a protobinary system appears. A wide-angle low-velocity outflow emerges from the circumbinary streams that enclose two protostars, while each protostar episodically drives high-velocity jets. Thus, the two high-velocity jets are surrounded by the low-velocity circumbinary outflow. The speed of the jets exceeds greater than or similar to 100 km s(-1). Although the jets have a collimated structure, they are swung back on the small scale and are tangled at the large scale due to the binary orbital motion. A circumstellar disk also appears around each protostar. In the early main accretion phase, the binary orbit is complicated, while the binary separation is within <30 au. For the first time, all the characteristics of protobinary systems recently observed with large telescopes are reproduced in a numerical simulation.

DOI： 10.3847/2041-8213/ab9d86

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/ab93d0

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/ab959e

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the Class I protostar L1489 IRS with the Atacama Millimeter/submillimeter Array (ALMA) in Band 6. The C¹⁸O J = 2-1 line emission shows flattened and non-axisymmetric structures in the same direction as its velocity gradient due to rotation. We discovered that the C¹⁸O emission shows dips at a radius of ∼200-300 au while the 1.3 mm continuum emission extends smoothly up to r ∼ 400 au. At the radius of the C¹⁸O dips, the rotational axis of the outer portion appears to be tilted by ∼15° from that of the inner component. Both the inner and outer components with respect to the C¹⁸O dips exhibit the r ^-0.5 Keplerian rotation profiles until r ∼ 600 au. These results not only indicate that a Keplerian disk extends up to ∼600 au but also that the disk is warped. We constructed a three-dimensional warped-disk model rotating at the Keplerian velocity, and demonstrated that the warped-disk model reproduces main observed features in the velocity channel maps and the PV diagrams. Such a warped-disk system can form by mass accretion from a misaligned envelope. We also discuss a possible disk evolution scenario based on comparisons of disk radii and masses between Class I and Class II sources.

DOI： 10.3847/1538-4357/ab8065

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the Class I protostar L1489 IRS with the Atacama Millimeter/submillimeter Array (ALMA) in Band 6. The (CO)-O-18 J = 2-1 line emission shows flattened and non-axisymmetric structures in the same direction as its velocity gradient due to rotation. We discovered that the (CO)-O-18 emission shows dips at a radius of similar to 200-300 au while the 1.3 mm continuum emission extends smoothly up to r similar to 400 au. At the radius of the (CO)-O-18 dips, the rotational axis of the outer portion appears to be tilted by similar to 15 degrees from that of the inner component. Both the inner and outer components with respect to the (CO)-O-18 dips exhibit the r(-0.5) Keplerian rotation profiles until r similar to 600 au. These results not only indicate that a Keplerian disk extends up to similar to 600 au but also that the disk is warped. We constructed a three-dimensional warped-disk model rotating at the Keplerian velocity, and demonstrated that the warped-disk model reproduces main observed features in the velocity channel maps and the PV diagrams. Such a warped-disk system can form by mass accretion from a misaligned envelope. We also discuss a possible disk evolution scenario based on comparisons of disk radii and masses between Class I and Class II sources.

DOI： 10.3847/1538-4357/ab8065

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using resistive magnetohydrodynamics simulations, the propagation of protostellar jets, the formation of circumstellar discs, and the configuration of magnetic fields are investigated from the pre-stellar cloud phase until ∼500 yr after protostar formation. As the initial state, we prepare magnetized rotating clouds, in which the rotation axis is misaligned with the global magnetic field by an angle θ0. We calculate the cloud evolution for nine models with different θ0 (= 0°, 5°, 10°, 30°, 45°, 60°, 80°, 85°, 90°). Our simulations show that there is no significant difference in the physical quantities of the protostellar jet, such as the mass and momentum, among the models except for the model with θ0 = 90°. On the other hand, the directions of the jet, disc normal, and magnetic field are never aligned with each other during the early phase of star formation except for the model with θ0 = 0°. Even when the rotation axis of the pre-stellar cloud is slightly inclined to the global magnetic field, the directions of the jet, disc normal, and local magnetic field differ considerably, and they randomly change over time. Our results indicate that it is very difficult to extract any information from the observations of the directions of the outflow, disc, and magnetic field at the scale of < 1000 au. Thus, we cannot use such observations to derive any restrictions on the star formation process.

DOI： 10.1093/mnras/stz3159

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the submillimeter continuum condensations SMM2, SMM4, SMM9, and SMM11 in the star-forming cluster Serpens Main using the Atacama Large Millimeter/submillimeter Array during Cycle 3 in the 1.3 mm continuum, ¹²CO J = 2 - 1, SO J _N = 6₅ - 5₄, and C¹⁸O J = 2 - 1 lines at an angular resolution of ∼0.″55 (240 au). Sixteen sources have been detected in the 1.3 mm continuum, which can be classified into three groups. Group 1 consists of six sources showing extended continuum emission and bipolar/monopolar ¹²CO outflows. Although all the Group 1 members are classified as Class 0 protostars, our observations suggest evolutionary trends among them in terms of ¹²CO outflow dynamical time, SO emission distribution, C¹⁸O fractional abundance, and continuum morphology. Group 2 consists of four sources associated with a continuum filamentary structure and no ¹²CO outflows. Central densities estimated from the 1.3 mm continuum intensity suggest that they are prestellar sources in a marginally Jeans unstable state. Group 3 consists of six Spitzer sources showing point-like 1.3 mm continuum emission and clumpy ¹²CO outflows. These features of Group 3 suggest envelope dissipation, preventing disk growth from the present size, r ≲ 60 au. The Group 3 members are protostars that may be precursors to the T Tauri stars associated with small disks at radii, of tens of astronomical units, identified in recent surveys.

DOI： 10.3847/1538-4357/ab5284

記述言語：その他  

Driving conditions of protostellar outflows in different star-forming environments

DOI： 10.1093/mnras/stz1079

記述言語：その他  

Origin of misalignments: protostellar jet, outflow, circumstellar disc, and magnetic field

DOI： 10.1093/mnras/stz740

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Both high-and low-velocity outflows are occasionally observed around a protostar by molecular line emission. The high-velocity component is called "extremely high velocity (EHV) flow, while the low-velocity component is simply referred to as "(molecular) outflow." This study reports a newly found EHV flow and outflow around MMS 5 in the Orion Molecular Cloud 3 observed with ALMA. In the observation, CO J = 2-1 emission traces both the EHV flow (vertical bar nu(LSR) - nu(sys)vertical bar similar or equal to 50-100 km s(-1)) and outflow (vertical bar nu(LSR) - nu(sys)vertical bar similar or equal to 10-50 km s(-1)). On the other hand, SiO J = 5-4 emission only traces the EHV flow. The EHV flow is collimated and located at the root of the V-shaped outflow. The CO outflow extends up to similar to 14,000 au with a position angle (P.A.) of similar to 79 degrees, and the CO redshifted EHV flow extends to similar to 11,000 au with a P.A. similar to 96 degrees. The EHV flow is smaller than the outflow, and the dynamical timescale of the EHV flow is shorter than that of the outflow by a factor of similar to 3. The flow driving mechanism is discussed based on the size, timescale, axis difference between the EHV flow and outflow, and periodicity of the knots. Our results are consistent with the nested wind scenario, although the jet entrainment scenario could not completely be ruled out.

DOI： 10.3847/1538-4357/aaf1b6

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We present the observational evidence of a pseudo-disc around the proto-brown dwarf Mayrit 1701117, the driving source of the large-scale HH 1165 jet. Our analysis is based on Atacama Large Millimeter/submillimeter Array ¹²CO (2-1) line and 1.37 mm continuum observations at an angular resolution of ∼0.4 arcsec. The pseudo-disc is a bright feature in the CO position-velocity diagram, elongated in a direction perpendicular to the jet axis, with a total (gas+dust) mass of ∼0.02 M_⊙, size of 165-192 au, and a velocity spread of ±2 km s^-1. The large velocity gradient is a combination of infalling and rotational motions, indicating a contribution from a pseudo-disc and an unresolved inner Keplerian disc. There is weak emission detected in the H₂CO (3-2) and N₂D⁺ (3-2) lines. H₂CO emission likely probes the inner Keplerian disc where CO is expected to be frozen, while N₂D⁺ possibly originates from an enhanced clump at the outer edge of the pseudo-disc. We have considered various models (core collapse, disc fragmentation, circumbinary disc) that can fit both the observed CO spectrum and the position-velocity offsets. The observed morphology, velocity structure, and the physical dimensions of the pseudo-disc are consistent with the predictions from the core collapse simulations for brown dwarf formation. From the best model fit, we can constrain the age of the proto-brown dwarf system to be ∼30 000-40 000 yr. A comparison of the H₂ column density derived from the CO line and 1.37 mm continuum emission indicates that only about 2 per cent of the CO is depleted from the gas phase.

DOI： 10.1093/mnras/stz1032

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/aae4dc

記述言語：英語   掲載種別：研究論文（学術雑誌）  

On the basis of various data such as ALMA, JVLA, Chandra, Herschel, and Spitzer, we confirmed that two protostellar candidates in Oph A are bona fide protostars or proto-brown dwarfs (proto-BDs) in extremely early evolutionary stages. Both objects are barely visible across infrared (IR; i.e., near-IR to far-IR) bands. The physical nature of the cores is very similar to that expected in first hydrostatic cores (FHSCs), objects theoretically predicted in the evolutionary phase prior to stellar core formation with gas densities of ∼10^11-12 cm^-3. This suggests that the evolutionary stage is close to the FHSC formation phase. The two objects are associated with faint X-ray sources, suggesting that they are in very early phase of stellar core formation with magnetic activity. In addition, we found the CO outflow components around both sources, which may originate from the young outflows driven by these sources. The masses of these objects are calculated to be ∼0.01-0.03 M _o from the dust continuum emission. These physical properties are consistent with that expected from the numerical model of forming brown dwarfs. These facts (the X-ray detection, CO outflow association, and FHSC-like spectral energy distributions) strongly indicate that the two objects are proto-BDs or will be in the very early phase of protostars, which will evolve to more massive protostars if they gain enough mass from their surroundings. The ages of these two objects are likely to be within ∼10³ years after the protostellar core (or second core) formation, taking into account the outflow dynamical times (≲500 years).

DOI： 10.3847/1538-4357/aae153

記述言語：その他  

Evolution of magnetic fields in collapsing star-forming clouds under different environments

DOI： 10.1093/mnras/sty046

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The driving mechanism of massive outflows observed in high-mass star-forming regions is investigated using three-dimensional magnetohydrodynamics (MHD) and protostellar evolution calculations. In our previous paper, we showed that the mass outflow rate depends strongly on the mass accretion rate on to the circumstellar disc around a high-mass protostar, and massive outflows may be driven by the magnetic effect in high-mass star-forming cores. In this study, in order to verify that the MHD disc wind is the primary driving mechanism of massive outflows, we quantitatively compare outflow properties obtained through simulations and observations. Since the outflows obtained through simulations are slightly younger than those obtained through observations, the time-integrated quantities of outflow mass, momentum, and kinetic energy are slightly smaller than those obtained through observations. On the other hand, time-derivative quantities of mass ejection rate, outflow momentum flux, and kinetic luminosity obtained through simulations are in very good agreement with those obtained through observations. This indicates that the MHD disc wind greatly contributes to the massive outflow driving from high-mass protostars, and the magnetic field might significantly control the high-mass star formation process.

DOI： 10.1093/mnras/stx3070

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Star formation in magnetically subcritical clouds is investigated using a three-dimensional non-ideal magnetohydrodynamic simulation. Since rapid cloud collapse is suppressed until the magnetic flux is sufficiently removed from the initially magnetically subcritical cloud by ambipolar diffusion, it takes ≳5-10 tff to form a protostar, where tff is the freefall time-scale of the initial cloud. The angular momentum of the star-forming cloud is efficiently transferred to the interstellar medium before the rapid collapse begins, and the collapsing cloud has a very low angular momentum. Unlike the magnetically supercritical case, no large-scale lowvelocity outflow appears in such a collapsing cloud due to the short lifetime of the first core. Following protostar formation, a very weak high-velocity jet, which has a small momentum and might disappear at a later time, is driven near the protostar, while the circumstellar disc does not grow during the early mass accretion phase. The results show that the star formation process in magnetically subcritical clouds is qualitatively different from that in magnetically supercritical clouds.

DOI： 10.1093/mnras/stx2589

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report ALMA observations in 0.87 mm continuum and (CO)-C-12 (J = 3-2) toward a very low-luminosity (< 0.1 L-circle dot) protostar, which is deeply embedded in one of the densest cores, MC27/ L1521F, in Taurus with an indication of multiple star formation in a highly dynamical environment. The beam size corresponds to similar to 20 au, and we have clearly detected blueshifted/ redshifted gas in (CO)-C-12 associated with the protostar. The spatial/ velocity distributions of the gas show there is a rotating disk with a size scale of similar to 10 au, a disk mass of similar to 10(-4) M-circle dot, and a central stellar mass of similar to 0.2 M-circle dot. The observed disk seems to be detached from the surrounding dense gas, although it is still embedded at the center of the core whose density is similar to 10(6) cm(-3). The current low-outflow activity and the very low luminosity indicate that the mass accretion rate onto the protostar is extremely low in spite of a very early stage of star formation. We may be witnessing the final stage of the formation of similar to 0.2 M-circle dot protostar. However, we cannot explain the observed low luminosity with the standard pre-main-sequence evolutionary track unless we assume cold accretion with an extremely small initial radius of the protostar (similar to 0.65 R.). These facts may challenge our current understanding of the low mass star formation, in particular the mass accretion process onto the protostar and the circumstellar disk.

DOI： 10.3847/1538-4357/aa8e9e

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the discovery of an extremely young protostar, SMM11, located in the associated submillimeter condensation in the Serpens Main cluster using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 3 at 1.3 mm and an angular resolution of similar to 0.'' 5 similar to 210 AU. SMM11 is a Class 0 protostar without any counterpart at 70 mu m or shorter wavelengths. The ALMA observations show 1.3 mm continuum emission associated with a collimated (CO)-C-12 bipolar outflow. Spitzer and Herschel data show that SMM11 is extremely cold (T-bol = 26 K) and faint (L-bol less than or similar to 0.9 L). We estimate the inclination angle of the outflow to be similar to 80 degrees, almost parallel to the plane of the sky, from simple fitting using a wind-driven-shell model. The continuum visibilities consist of Gaussian and power-law components, suggesting a spherical envelope with a radius of similar to 600 au around the protostar. The estimated low (CO)-O-18 abundance, X((CO)-O-18). =. 1.5-3 x 10(-10), is also consistent with its youth. The high outflow velocity, a few 10 km s(-1) at a few 1000 au, is much higher than theoretical simulations of first hydrostatic cores, and we suggest that SMM11 is a transitional object right after the second collapse of the first core.

DOI： 10.3847/2041-8213/aa9701

記述言語：その他   掲載種別：研究論文（学術雑誌）  

We present observational results of the submillimeter H2O and SiO lines<br />
toward a candidate high-mass young stellar object Orion Source I using ALMA.<br />
The spatial structures of the high excitation lines at lower-state energies of<br />
>2500 K show compact structures consistent with the circumstellar disk and/or<br />
base of the northeast-southwest bipolar outflow with a 100 au scale. The<br />
highest excitation transition, the SiO (v=4) line at band 8, has the most<br />
compact structure. In contrast, lower-excitation transitions are more extended<br />
than 200 au tracing the outflow. Almost all the line show velocity gradients<br />
perpendicular to the outflow axis suggesting rotation motions of the<br />
circumstellar disk and outflow. While some of the detected lines show broad<br />
line profiles and spatially extended emission components indicative of thermal<br />
excitation, the strong H2O lines at 321 GHz, 474 GHz, and 658 GHz with<br />
brightness temperatures of >1000 K show clear signatures of maser action.

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have recently observed the Class 0/I protostar L1527 IRS using the Atacama Large Millimeter/submillimeter Array (ALMA) during its Cycle 1 in 220 GHz dust continuum and (CO)-O-18 (J = 2-1) line emissions with a similar to 2 times higher angular resolution (similar to 0 ''.5) and similar to 4 times better sensitivity than our ALMA Cycle 0 observations. Continuum emission shows elongation perpendicular to the associated outflow, with a deconvolved size of 0 ''.53 x 0 ''.15. (CO)-O-18 emission shows similar elongation, indicating that both emissions trace the disk and the flattened envelope surrounding the protostar. The velocity gradient of the (CO)-O-18 emission along the elongation due to rotation of the disk/envelope system is reanalyzed, identifying Keplerian rotation proportional to r(-0.5) more clearly than the Cycle 0 observations. The Keplerian-disk radius and the dynamical stellar mass are kinematically estimated to be similar to 74 au and similar to 0.45 M-circle dot, respectively. The continuum visibility is fitted by models without any annulus averaging, revealing that the disk is in hydrostatic equilibrium. The best-fit model also suggests a density jump by a factor of similar to 5 between the disk and the envelope, suggesting that disks around protostars can be geometrically distinguishable from the envelope from a viewpoint of density contrast. Importantly, the disk radius geometrically identified with the density jump is consistent with the kinematically estimated radius. Possible origin of the density jump due to the mass accretion from the envelope to the disk is discussed. (CO)-O-18 observations can be reproduced by the same geometrical structures derived from the dust observations, with possible (CO)-O-18 freeze-out and localized (CO)-O-18 desorption.

DOI： 10.3847/1538-4357/aa8264

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The relation between the mass accretion rate on to the circumstellar disc and the rate of mass ejection by magnetically driven winds is investigated using three-dimensional magnetohydrodynamics simulations. Using a spherical cloud core with a varying ratio of thermal to gravitational energy, which determines the mass accretion rate on to the disc, to define the initial conditions, the outflow propagation for approximately 10(4) yr after protostar formation is then calculated for several cloud cores. The mass ejection rate and accretion rate are comparable only when the magnetic energy of the initial cloud core is comparable to the gravitational energy. Consequently, in strongly magnetized clouds a higher mass accretion rate naturally produces both massive protostars and massive outflows. The simulated outflow mass, momentum, kinetic energy and momentum flux agree well with observations, indicating that massive stars form through the same mechanism as low-mass stars but require a significantly strong magnetic field to launch massive outflows.

DOI： 10.1093/mnras/stx893

記述言語：英語   掲載種別：研究論文（学術雑誌）  

One of the outstanding problems in star formation theory concerns the transfer of angular momentum so that mass can accrete onto a newly born young stellar object (YSO). From a theoretical standpoint, outflows and jets are predicted to play an essential role in the transfer of angular momentum(1-4) and their rotations have been reported for both low-(5) and highmass(6,7) YSOs. However, little quantitative discussion on outflow launching mechanisms has been presented for high-mass YSOs due to a lack of observational data. Here we present a clear signature of rotation in the bipolar outflow driven by Orion Source I, a high-mass YSO candidate, using the Atacama Large Millimeter/Submillimeter Array (ALMA). A rotational transition of silicon monoxide ((SiO)-O-18) reveals a velocity gradient perpendicular to the outflow axis, which is consistent with that of the circumstellar disk traced by a high excitation water line. The launching radii and outward velocity of the outflow are estimated to be >10 au and 10 km s(-1), respectively. These parameters rule out the possibility that the observed outflow is produced by the entrainment of a high-velocity jet(8), and that contributions from the stellar wind(9) or X-wind(10), which have smaller launching radii, are significant in the case of Source I. Thus these results provide convincing evidence of a rotating outflow directly driven by the magneto-centrifugal disk wind launched by a high-mass YSO candidate(6,11).

DOI： 10.1038/s41550-017-0146

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the formation of circumstellar disks and outflows subsequent to the collapse of molecular cloud cores with the magnetic field and turbulence. Numerical simulations are performed by using an adaptive mesh refinement to follow the evolution up to similar to 1000. years after the formation of a protostar. In the simulations, circumstellar disks are formed around the protostars; those in magnetized models are considerably smaller than those in nonmagnetized models, but their size increases with time. The models with stronger magnetic fields tend. to produce smaller disks. During evolution in the magnetized models, the mass ratios of a disk to a protostar is approximately constant at similar to 1%-10%. The circumstellar disks are aligned according to their angular momentum, and the outflows accelerate along the magnetic field on the 10-100 au scale; this produces a disk that is misaligned with the outflow. The outflows are classified into two types: a magnetocentrifugal wind and a spiral flow. In the latter, because of the geometry, the axis of rotation is misaligned with the magnetic field. The magnetic field has an internal structure in the cloud cores, which also causes misalignment between the outflows and the magnetic field on the scale of the cloud core. The distribution of the angular momentum vectors in a core also has a non-monotonic internal structure. This should create a time-dependent accretion of angular momenta onto the circumstellar disk. Therefore, the circumstellar disks are expected to change their orientation as well as their sizes in the long-term evolutions.

DOI： 10.3847/1538-4357/aa6a1c

記述言語：英語   掲載種別：研究論文（学術雑誌）  

DOI： 10.3847/1538-4357/836/1/148

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We study formation and long-term evolution of a circumstellar disk in a collapsing molecular cloud core using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues, and its radius becomes as large as 200 au toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum in the highly resistive disk. Although the spiral arms disappear in a few rotations as expected in a classical theory, new spiral arms form recurrently as the disk, soon becoming unstable again by gas accretion. Such recurrent spiral arms persist throughout the Class-0 and I phases. We then perform synthetic observations and compare our model with a recent high-resolution observation of a young stellar object Elias 2-27, whose circumstellar disk has grand-design spiral arms. We find good agreement between our theoretical model and the observation. Our model suggests that the grand-design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

DOI： 10.3847/2041-8213/835/1/L11

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have measured continuum flux densities of a high-mass protostar candidate, a radio source. I in the Orion. KL region (Orion Source I) using the Atacama Large Millimeter/Submillimeter Array (ALMA) at band. 8 with an angular resolution of 0 ''.1. The continuum emission at 430, 460, and 490 GHz associated with Source. I shows an elongated structure along the northwest-southeast direction perpendicular to the so-called low-velocity bipolar outflow. The deconvolved size of the continuum source, 90 au x 20 au, is consistent with those reported previously at other millimeter/submillimeter wavelengths. The flux density can be well fitted to the optically thick blackbody spectral energy distribution, and the brightness temperature is evaluated to be 700-800 K. It is much lower than that in the case of proton-electron or H-free-free radiations. Our data are consistent with the latest ALMA results by Plambeck & Wright, in which the continuum emission was proposed to arise from the edge-on circumstellar disk via thermal dust emission, unless the continuum source consists of an unresolved structure with a smaller beam filling factor.

DOI： 10.3847/1538-4357/833/2/238

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Disc formation in strongly magnetized cloud cores is investigated using a three-dimensional magnetohydrodynamic simulation with a focus on the effects of the initial cloud stability and the mass accretion rate. The initial cloud stability greatly alters the disc formation process even for prestellar clouds with the same mass-to-flux ratio. A high mass accretion rate on to the disc-forming region is realized in initially unstable clouds, and a large angular momentum is introduced into the circumstellar region in a short time. The region around the protostar has both a thin infalling envelope and a weak magnetic field, which both weaken the effect of magnetic braking. The growth of the rotation-supported disc is promoted in such unstable clouds. Conversely, clouds in an initially near-equilibrium state show lower accretion rates of mass and angular momentum. The angular momentum is transported to the outer envelope before protostar formation. After protostar formation, the circumstellar region has a thick infalling envelope and a strong magnetic field that effectively brakes the disc. As a result, disc formation is suppressed when the initial cloud is in a nearly stable state. The density distribution of the initial cloud also affects the disc formation process. Disc growth strongly depends on the initial conditions when the prestellar cloud has a uniform density, whereas there is no significant difference in the disc formation process in prestellar clouds with non-uniform densities.

DOI： 10.1093/mnras/stw2256

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We present the results of ALMA observations of dust continuum emission and molecular rotational lines toward a dense core MC27 (aka L1521F) in Taurus, which is considered to be at a very early stage of star formation. The detailed column density distributions on size scales from a few tens to similar to 10,000 AU are revealed by combining the ALMA (12m array + 7 m array) data with the published/unpublished single-dish data. The high angular resolution observations at 0.87 mm with a synthesized beam size of similar to 0 ''.74 x 0 ''.32 reveal that a protostellar source, MMS-1, is not spatially resolved and lacks associated gas emission, while a starless high-density core, MMS-2, has substructures in both dust and molecular emission. The averaged radial column density distribution of the inner part of MC27/L1521F (r less than or similar to 3000 AU) is N-H2 similar to r(-0.4), clearly flatter than that of the outer part, similar to r(-1.0). The complex velocity/spatial structure obtained with previous ALMA observations is located inside the inner flatter region, which may reflect the dynamical status of the dense core.

DOI： 10.3847/0004-637X/826/1/26

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have observed the Class I protostar TMC-1A with the Atacama Millimeter/submillimeter Array (ALMA) in the emissions of (CO)-C-12 and (CO)-O-18 (J = 2-1). and 1.3. mm dust continuum. Continuum emission with a deconvolved size of 0 ''.50 x 0 ''.37, perpendicular to the (CO)-C-12 outflow, is detected. It most likely traces a circumstellar disk around TMC-1A, as previously reported. In contrast, a more extended structure is detected in (CO)-O-18, although it is still elongated with a deconvolved size of 3 ''.3 x 2 ''.2, indicating that (CO)-O-18 traces mainly a flattened envelope surrounding the disk and the central protostar. (CO)-O-18 shows a clear velocity gradient perpendicular to the outflow at higher velocities, indicative of rotation, while an additional velocity gradient along the outflow is found at lower velocities. The radial profile of the rotational velocity is analyzed in detail, finding that it is given as a power. law proportional to r(-a) with an index of similar to 0.5 at higher velocities. This indicates that the rotation at higher velocities can be explained as Keplerian rotation orbiting a protostar with a dynamical mass of 0.68 M-circle dot (inclination corrected). The additional velocity gradient of (CO)-O-18 along the outflow is considered to be mainly infall motions in the envelope. Position-velocity diagrams made from models consisting of an infalling envelope and a Keplerian disk are compared with the observations, revealing that the observed infall velocity is similar to 0.3 times smaller than the free-fall velocity yielded by the dynamical mass of the protostar. Magnetic fields could be responsible for the slow infall velocity. A possible scenario of Keplerian disk formation is discussed.

DOI： 10.1088/0004-637X/812/1/27

記述言語：その他   掲載種別：研究論文（学術雑誌）  

DOI： 10.1088/2041-8205/810/2/L26

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The main accretion phase of star formation is investigated in clouds with different metallicities in the range 0 <= Z <= Z(circle dot), resolving the protostellar radius. Starting from a near-equilibrium prestellar cloud, we calculate the cloud evolution up to similar to 100 yr after the first protostar forms. Star formation differs considerably between clouds with lower (Z <= 10(-4) Z(circle dot)) and higher (Z > 10(-4) Z(circle dot)) metallicities. Fragmentation frequently occurs and many protostars appear without a stable circumstellar disc in lower-metallicity clouds. In these clouds, although protostars mutually interact and some are ejected from the cloud centre, many remain as a small stellar cluster. In contrast, higher-metallicity clouds produce a single protostar surrounded by a nearly stable rotation-supported disc. In these clouds, although fragmentation occasionally occurs in the disc, the fragments migrate inwards and finally fall on to the central protostar. The difference in cloud evolution is due to different thermal evolutions and mass accretion rates. The thermal evolution of the cloud determines the emergence and lifetime of the first core. The first core develops prior to the formation of a protostar in higher-metallicity clouds, whereas no (obvious) first core appears in lower-metallicity clouds. The first core evolves into a circumstellar disc with a spiral pattern, which effectively transfers the angular momentum outwards and suppresses frequent fragmentation. In lower-metallicity clouds, the higher mass accretion rate increases the disc surface density within a very short time, rendering the disc unstable to self-gravity and inducing vigorous fragmentation.

DOI： 10.1093/mnras/stu2633

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The transport of angular momentum by magnetic fields is a crucial physical process in the formation and evolution of stars and disks. Because the ionization degree in star-forming clouds is extremely low, nonideal magnetohydrodynamic (MHD) effects such as ambipolar diffusion and ohmic dissipation work strongly during protostellar collapse. These effects have significant impacts in the early phase of star formation as they redistribute magnetic flux and suppress angular momentum transport by magnetic fields. We perform three-dimensional nested-grid radiation magnetohydrodynamic simulations including ohmic dissipation and ambipolar diffusion. Without these effects, magnetic fields transport angular momentum so efficiently that no rotationally supported disk is formed even after the second collapse. Ohmic dissipation works only in a relatively high density region within the first core and suppresses angular momentum transport, enabling formation of a very small rotationally supported disk after the second collapse. With both ohmic dissipation and ambipolar diffusion, these effects work effectively in almost the entire region within the first core and significant magnetic flux loss occurs. As a result, a rotationally supported disk is formed even before a protostellar core forms. The size of the disk is still small, about 5 AU at the end of the first core phase, but this disk will grow later as gas accretion continues. Thus, the nonideal MHD effects can resolve the so-called magnetic braking catastrophe while keeping. the disk size small in the early phase, which is implied from recent interferometric observations.

DOI： 10.1088/0004-637X/801/2/117

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of the (CO)-O-18 (J = 2-1), SO (J(N) = 6(5)-5(4)), and the 1.3 mm dust continuum toward L1527 IRS, a class 0 solar-type protostar surrounded by an infalling and rotating envelope. (CO)-O-18 emission shows strong redshifted absorption against the bright continuum emission associated with L1527 IRS, strongly suggesting infall motions in the (CO)-O-18 envelope. The (CO)-O-18 envelope also rotates with a velocity mostly proportional to r(-1), where r is the radius, whereas the rotation profile at the innermost radius (similar to 54 AU) may be shallower than r(-1), suggestive of formation of a Keplerian disk around the central protostar of similar to 0.3 M-circle dot in dynamical mass. SO emission arising from the inner part of the (CO)-O-18 envelope also shows rotation in the same direction as the (CO)-O-18 envelope. The rotation is, however, rigid-body-like, which is very different from the differential rotation shown by (CO)-O-18. In order to explain the line profiles and the position-velocity (PV) diagrams of (CO)-O-18 and SO observed, simple models composed of an infalling envelope surrounding a Keplerian disk of 54 AU in radius orbiting a star of 0.3 M-circle dot are examined. It is found that in order to reproduce characteristic features of the observed line profiles and PV diagrams, the infall velocity in the model has to be smaller than the free-fall velocity yielded by a star of 0.3 M-circle dot. Possible reasons for the reduced infall velocities are discussed.

DOI： 10.1088/0004-637X/796/2/131

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A protostellar jet and outflow are calculated for similar to 270 yr following the protostar formation using a three-dimensional magnetohydrodynamics simulation, in which both the protostar and its parent cloud are spatially resolved. A high-velocity (similar to 100 km s(-1)) jet with good collimation is driven near the disk's inner edge, while a low-velocity (less than or similar to 10 km s(-1)) outflow with a wide opening angle appears in the outer-disk region. The high-velocity jet propagates into the low-velocity outflow, forming a nested velocity structure in which a narrow high-velocity flow is enclosed by a wide low-velocity flow. The low-velocity outflow is in a nearly steady state, while the high-velocity jet appears intermittently. The time-variability of the jet is related to the episodic accretion from the disk onto the protostar, which is caused by gravitational instability and magnetic effects such as magnetic braking and magnetorotational instability. Although the high-velocity jet has a large kinetic energy, the mass and momentum of the jet are much smaller than those of the low-velocity outflow. A large fraction of the infalling gas is ejected by the low-velocity outflow. Thus, the low-velocity outflow actually has a more significant effect than the high-velocity jet in the very early phase of the star formation.

DOI： 10.1088/2041-8205/796/1/L17

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We have conducted ALMA observations in the 1.3 mm continuum and (CO)-C-12 (2-1), (CO)-O-18 (2-1), and SO (5(6)-4(5)) lines toward L1489 IRS, a Class I protostar surrounded by a Keplerian disk and an infalling envelope. The Keplerian disk is clearly identified in the (CO)-C-12 and (CO)-O-18 emission, and its outer radius (similar to 700 AU) and mass (similar to 0.005 M-circle dot) are comparable to those of disks around T Tauri stars. The protostellar mass is estimated to be 1.6 M-circle dot with the inclination angle of 66 degrees. In addition to the Keplerian disk, there are blueshifted and redshifted off-axis protrusions seen in the (CO)-O-18 emission pointing toward the north and the south, respectively, adjunct to the middle part of the Keplerian disk. The shape and kinematics of these protrusions can be interpreted as streams of infalling flows with a conserved angular momentum following parabolic trajectories toward the Keplerian disk, and the mass infalling rate is estimated to be similar to 5 x 10(-7) M-circle dot yr(-1). The specific angular momentum of the infalling flows (similar to 2.5 x 10(-3) km s(-1) pc) is comparable to that at the outer radius of the Keplerian disk (similar to 4.8 x 10(-3) km s(-1) pc). The SO emission is elongated along the disk major axis and exhibits a linear velocity gradient along the axis, which is interpreted to mean that the SO emission primarily traces a ring region in the flared Keplerian disk at radii of similar to 250-390 AU. The local enhancement of the SO abundance in the ring region can be due to the accretion shocks at the centrifugal radius where the infalling flows fall onto the disk. Our ALMA observations unveiled both the Keplerian disk and the infalling gas onto the disk, and the disk can further grow by accreting material and angular momenta from the infalling gas.

DOI： 10.1088/0004-637X/793/1/1

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Starless dense cores eventually collapse dynamically, forming protostars inside them, and the physical properties of the cores determine the nature of the forming protostars. We report ALMA observations of dust continuum emission and molecular rotational lines toward MC27 or L1521F, which is considered to be very close to the first protostellar core phase. We found a few starless high-density cores, one of which has a very high density of similar to 10(7) cm(-3), within a region of several hundred AU around a very low-luminosity protostar detected by Spitzer. A very compact bipolar outflow with a dynamical timescale of a few hundred years was found toward the protostar. The molecular line observation shows several cores with an arc-like structure, possibly due to the dynamical gas interaction. These complex structures revealed in the present observations suggest that the initial condition of star formation is highly dynamical in nature, which is considered to be a key factor in understanding fundamental issues of star formation such as the formation of multiple stars and the origin of the initial mass function of stars.

DOI： 10.1088/2041-8205/789/1/L4

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10(-2)-10(6) m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

DOI： 10.1088/0004-637X/784/2/109

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation of a circumstellar disc in collapsing cloud cores is investigated with three-dimensional magnetohydrodynamic simulations. We prepare four types of initial cloud having different density profiles and calculate their evolution with or without a sink. To investigate the effect of magnetic dissipation on disc formation, Ohmic dissipation is considered in some models. Calculations show that disc formation is very sensitive to both the initial cloud configuration and the sink treatment. The disc size considerably differs in clouds with different density profiles even when the initial clouds have almost the same mass-to-flux ratio. Only a very small disc (similar to 10 au in size) appears in clouds with a uniform density profile, whereas a large disc (similar to 100 au in size) forms in clouds with a Bonnor-Ebert density profile. In addition, a large sink accretion radius numerically impedes disc formation during the main accretion phase and tends to foster the misleading notion that disc formation is completely suppressed by magnetic braking. The protostellar outflow is also greatly affected by the sink properties. A sink accretion radius of less than or similar to 1 au and sink threshold density of greater than or similar to 10(13) cm(-3) are necessary for investigating disc formation during the main accretion phase.

DOI： 10.1093/mnras/stt2343

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the results of our three-dimensional radiation hydrodynamics simulation of collapsing unmagnetized molecular cloud cores. We investigate the formation and evolution of the circumstellar disc and the clumps formed by disc fragmentation. Our simulation shows that disc fragmentation occurs in the early phase of circumstellar disc evolution and many clumps form. The clump can be represented by a polytrope sphere of index n similar to 3 and n greater than or similar to 4 at central temperature T-c less than or similar to 100 K and T-c greater than or similar to 100 K, respectively. We demonstrate, numerically and theoretically, that the maximum mass of the clump, beyond which it inevitably collapses, is similar to 0.03 M-circle dot. The entropy of the clump increases during its evolution, implying that evolution is chiefly determined by mass accretion from the disc rather than by radiative cooling. Although most of the clumps rapidly migrate inward and finally fall on to the protostar, a few clumps remain in the disc. The central density and temperature of the surviving clump rapidly increase and the clump undergoes a second collapse within 1000-2000 years after its formation. In our simulation, three second cores of masses 0.2 M-circle dot, 0.15 M-circle dot and 0.06 M-circle dot formed. These are protostars or brown dwarfs rather than protoplanets. For the clumps to survive as planetary-mass objects, the rapid mass accretion should be prevented by some mechanisms.

DOI： 10.1093/mnras/stt1684

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the formation process of self-gravitating protoplanetary disks in unmagnetized molecular clouds. The angular momentum is redistributed by the action of gravitational torques in the massive disk during its early formation. We develop a simplified one-dimensional accretion disk model that takes into account the infall of gas from the envelope onto the disk and the transfer of angular momentum in the disk with an effective viscosity. First we evaluate the gas accretion rate from the cloud core onto the disk by approximately estimating the effects of gas pressure and gravity acting on the cloud core. We formulate the effective viscosity as a function of the Toomre Q parameter that measures the local gravitational stability of the rotating thin disk. We use a function for viscosity that changes sensitively with Q when the disk is gravitationally unstable. We find a strong self-regulation mechanism in the disk evolution. During the formation stage of protoplanetary disks, the evolution of the surface density does not depend on the other details of the modeling of effective viscosity, such as the prefactor of the viscosity coefficient. Next, to verify our model, we compare the time evolution of the disk calculated with our formulation with that of three-dimensional hydrodynamical simulations. The structures of the resultant disks from the one-dimensional accretion disk model agree well with those of the three-dimensional simulations. Our model is a useful tool for the further modeling of chemistry, radiative transfer, and planet formation in protoplanetary disks.

DOI： 10.1088/0004-637X/770/1/71

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the formation and evolution of circumstellar discs in turbulent cloud cores until several 10(4) yr after protostar formation using smoothed particle hydrodynamics (SPH) calculations. The formation and evolution process of circumstellar disc in turbulent cloud cores differs substantially from that in rigidly rotating cloud cores. In turbulent cloud cores, a filamentary structure appears before the protostar formation and the protostar forms in the filament. If the turbulence is initially sufficiently strong, the remaining filament twists around the protostar and directly becomes a rotation-supported disc. Upon formation, the disc orientation is generally misaligned with the angular momentum of its host cloud core and it dynamically varies during the main accretion phase, even though the turbulence is weak. This is because the angular momentum of the entire cloud core is mainly determined by the large-scale velocity field whose wavelength is comparable to the cloud scale, whereas the angular momentum of the disc is determined by the local velocity field where the protostar forms and these two velocity fields do not correlate with each other. In the case of disc evolution in a binary or multiple stars, the discs are misaligned with each other at least during the main accretion phase, because there is no correlation between the velocity fields around the position where each protostar forms. In addition, each disc is also misaligned with the binary orbital plane. Such misalignment can explain the recent observations of misaligned discs and misaligned protostellar outflows.

DOI： 10.1093/mnras/sts111

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We report the first three-dimensional radiation magnetohydrodynamic (RMHD) simulations of protostellar collapse with and without Ohmic dissipation. We take into account many physical processes required to study star formation processes, including a realistic equation of state. We follow the evolution from molecular cloud cores until protostellar cores are formed with sufficiently high resolutions without introducing a sink particle. The physical processes involved in the simulations and adopted numerical methods are described in detail. We can calculate only about one year after the formation of the protostellar cores with our direct three-dimensional RMHD simulations because of the extremely short timescale in the deep interior of the formed protostellar cores, but successfully describe the early phase of star formation processes. The thermal evolution and the structure of the first and second (protostellar) cores are consistent with previous one-dimensional simulations using full radiation transfer, but differ considerably from preceding multi-dimensional studies with the barotropic approximation. The protostellar cores evolve virtually spherically symmetric in the ideal MHD models because of efficient angular momentum transport by magnetic fields, but Ohmic dissipation enables the formation of the circumstellar disks in the vicinity of the protostellar cores as in previous MHD studies with the barotropic approximation. The formed disks are still small (less than 0.35 AU) because we simulate only the earliest evolution. We also confirm that two different types of outflows are naturally launched by magnetic fields from the first cores and protostellar cores in the resistive MHD models.

DOI： 10.1088/0004-637X/763/1/6

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The configuration and evolution of the magnetic field in star-forming cores are investigated in order to directly compare simulations and observations. We prepare four different initial clouds having different magnetic field strengths and rotation rates, in which magnetic field lines are aligned/misaligned with the rotation axis. First, we calculate the evolution of such clouds from the prestellar stage until long after protostar formation. Then, we calculate the polarization of thermal dust emission expected from the simulation data. We create polarization maps with arbitrary viewing angles and compare them with observations. Using this procedure, we confirmed that the polarization distribution projected on the celestial plane strongly depends on the viewing angle of the cloud. Thus, by comparing the observations with the polarization map predicted by the simulations, we can roughly determine the angle between the direction of the global magnetic field and the line of sight. The configuration of the polarization vectors also depends on the viewing angle. We find that an hourglass configuration of magnetic field lines is not always realized in a collapsing cloud when the global magnetic field is misaligned with the cloud rotation axis. Depending on the viewing angle, an S-shaped configuration of the magnetic field (or the polarization vectors) appears early in the protostellar accretion phase. This indicates that not only the magnetic field but also the cloud rotation affects the dynamical evolution of such a cloud. In addition, by comparing the simulated polarization with actual observations, we can estimate properties of the host cloud such as the evolutionary stage, magnetic field strength, and rotation rate.

DOI： 10.1088/0004-637X/761/1/40

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We measured polarized dust emission at 350 mu m toward the high-mass star-forming massive dense clump IRAS 20126+4104 using the SHARC II Polarimeter, SHARP, at the Caltech Submillimeter Observatory. Most of the observed magnetic field vectors agree well with magnetic field vectors obtained from a numerical simulation for the case when the global magnetic field lines are inclined with respect to the rotation axis of the dense clump. The results of the numerical simulation show that rotation plays an important role on the evolution of the massive dense clump and its magnetic field. The direction of the cold CO 1-0 bipolar outflow is parallel to the observed magnetic field within the dense clump as well as the global magnetic field, as inferred from optical polarimetry data, indicating that the magnetic field also plays a critical role in an early stage of massive star formation. The large-scale Keplerian disk of the massive (proto) star rotates in an almost opposite sense to the clump's envelope. The observed magnetic field morphology and the counterrotating feature of the massive dense clump system provide hints to constrain the role of magnetic fields in the process of high-mass star formation.

DOI： 10.1088/2041-8205/750/2/L29

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate gas accretion flow onto a circumplanetary disk from a protoplanetary disk in detail by using high-resolution three-dimensional nested-grid hydrodynamic simulations, in order to provide a basis of formation processes of satellites around giant planets. Based on detailed analyses of gas accretion flow, we find that most of gas accretion onto circumplanetary disks occurs nearly vertically toward the disk surface from high altitude, which generates a shock surface at several scale heights of the circumplanetary disk. The gas that has passed through the shock surface moves inward because its specific angular momentum is smaller than that of the local Keplerian rotation, while gas near the midplane in the protoplanetary disk cannot accrete to the circumplanetary disk. Gas near the midplane within the planet's Hill sphere spirals outward and escapes from the Hill sphere through the two Lagrangian points L-1 and L-2. We also analyze fluxes of accreting mass and angular momentum in detail and find that the distributions of the fluxes onto the disk surface are well described by power-law functions and that a large fraction of gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times the Hill radius. The nature of power-law functions indicates that, other than the outer edge, there is no specific radius where gas accretion is concentrated. These source functions of mass and angular momentum in the circumplanetary disk would provide us with useful constraints on the structure and evolution of the circumplanetary disk, which is important for satellite formation.

DOI： 10.1088/0004-637X/747/1/47

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The effects of a protostellar outflow on the star formation in a single cloud core are investigated by three-dimensional resistive magnetohydrodynamic (MHD) simulations. Starting from the pre-stellar cloud core, the star formation process is calculated until the end of the main accretion phase. In the calculations, the mass of the pre-stellar cloud is parametrized. During the star formation, the protostellar outflow is driven by the circumstellar disc. The outflow extends also in the transverse direction until its width becomes comparable to the initial cloud scale, and thus the outflow has a wide opening angle of greater than or similar to 40 degrees. As a result, the protostellar outflow sweeps up a large fraction of the infalling material and ejects it into the interstellar space. The outflow can eject at most over half of the host cloud mass, significantly decreasing the star formation efficiency. The outflow power is stronger in clouds with a greater initial mass. Thus, the protostellar outflow effectively suppresses the star formation efficiency in a massive cloud. The outflow weakens significantly and disappears in several free-fall time-scales of the initial cloud after the cloud begins to collapse. The natal pre-stellar core influences the lifetime and size of the outflow. At the end of the main accretion phase, a massive circumstellar disc comparable in mass to the protostar remains. Calculations show that similar to 26-54 per cent of the initial cloud mass is converted into the protostar and similar to 8-40 per cent remains in the circumstellar disc, while similar to 8-49 per cent can be ejected into the interstellar space by the protostellar outflow. Therefore, the protostellar outflow can decrease the star formation efficiency to similar to 50 per cent at the maximum.

DOI： 10.1111/j.1365-2966.2011.20336.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We performed hydrodynamical simulations to investigate the formation and evolution of protostars and circumstellar discs from the pre-stellar cloud. As the initial state, we adopted the molecular cloud core with two non-dimensional parameters representing the thermal and rotational energies. With these parameters, we derived 17 models and calculated the cloud evolution similar to 10(4) yr after the protostar formation. We found that early evolution of the star-disc system can be qualitatively classified into four modes: the massive-disc, early-fragmentation, late-fragmentation, and protostar-dominant modes. In the 'massive-disc mode', to which the majority of models belong, the disc mass is greater than the protostellar mass for over 10(4) yr and no fragmentation occurs in the circumstellar disc. The collapsing cloud shows fragmentation before the protostar formation in the 'early-fragmentation mode'. The circumstellar disc shows fragmentation after the protostar formation in the 'late-fragmentation mode', in which the secondary star gains most of its mass from the circumstellar disc after fragmentation and has a mass comparable to that of the primary star. The protostellar mass rapidly increases and exceeds the circumstellar disc mass in the 'protostar-dominant mode'. This mode appears only when the initial molecular cloud core has a very small rotational energy. Comparison of our results with observations indicates that the majority of protostars have a fairly massive disc during the main accretion phase: the circumstellar disc mass is comparable to or more massive than the protostar mass. It is expected that such a massive disc promotes gas-giant formation by gravitational instability in a subsequent evolutionary stage.

DOI： 10.1111/j.1365-2966.2011.19081.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Using resistive magnetohydrodynamics simulation, we consider circumstellar disk formation in a strongly magnetized cloud. As the initial state, an isolated cloud core embedded in a low-density interstellar medium with a uniform magnetic field was adopted. The cloud evolution was calculated until almost all gas inside the initial cloud fell onto either the circumstellar disk or a protostar, and a part of the gas was ejected into the interstellar medium by the protostellar outflow driven by the circumstellar disk. In the early main accretion phase, the disk size is limited to similar to 10 AU because the angular momentum of the circumstellar disk is effectively transferred by both magnetic braking and the protostellar outflow. In the later main accretion phase, however, the circumstellar disk grows rapidly and exceeds greater than or similar to 100 AU by the end of the main accretion phase. This rapid growth of the circumstellar disk is caused by depletion of the infalling envelope, while magnetic braking is effective when the infalling envelope is more massive than the circumstellar disk. The infalling envelope cannot brake the circumstellar disk when the latter is more massive than the former. In addition, the protostellar outflow weakens and disappears in the later main accretion phase, because the outflow is powered by gas accretion onto the circumstellar disk. Although the circumstellar disk formed in a magnetized cloud is considerably smaller than that in an unmagnetized cloud, a circumstellar disk exceeding 100 AU can form even in a strongly magnetized cloud.

DOI： 10.1093/pasj/63.3.555

記述言語：英語  

DOI： 10.1088/2041-8205/732/1/L18

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation and evolution of a circumstellar disk in magnetized cloud cores are investigated from a prestellar core stage until similar to 10(4) yr after protostar formation. In the circumstellar disk, fragmentation first occurs due to gravitational instability in a magnetically inactive region, and substellar-mass objects appear. The substellar-mass objects lose their orbital angular momenta by gravitational interaction with the massive circumstellar disk and finally fall onto the protostar. After this fall, the circumstellar disk increases its mass by mass accretion and again induces fragmentation. The formation and falling of substellar-mass objects are repeated in the circumstellar disk until the end of the main accretion phase. In this process, the mass of the fragments remains small, because the circumstellar disk loses its mass by fragmentation and subsequent falling of fragments before it becomes very massive. In addition, when fragments orbit near the protostar, they disturb the inner disk region and promote mass accretion onto the protostar. The orbital motion of substellar-mass objects clearly synchronizes with the time variation of the accretion luminosity of the protostar. Moreover, as the objects fall, the protostar shows a strong brightening for a short duration. The intermittent protostellar outflows are also driven by the circumstellar disk whose magnetic field lines are highly tangled owing to the orbital motion of fragments. The time-variable protostellar luminosity and intermittent outflows may be a clue for detecting planetary-mass objects in the circumstellar disk.

DOI： 10.1088/0004-637X/729/1/42

記述言語：英語   掲載種別：研究論文（学術雑誌）  

A first adiabatic core is a transient object formed in the early phase of star formation. The observation of a first core is believed to be difficult because of its short lifetime and low luminosity. On the basis of radiation hydrodynamic simulations, we propose a novel theoretical model of first cores, the Exposed Long-lifetime First core (ELF). In the very low-mass molecular core, the first core evolves slowly and lives longer than 10,000 years because the accretion rate is considerably low. The evolution of ELFs is different from that of ordinary first cores because radiation cooling has a significant effect there. We also carry out a radiation-transfer calculation of dust-continuum emission from ELFs to predict their observational properties. ELFs have slightly fainter but similar spectral energy distributions to ordinary first cores in radio wavelengths, therefore they can be observed. Although the probabilities that such low-mass cores become gravitationally unstable and start to collapse are low, we still can expect that a considerable number of ELFs can be formed because there are many low-mass molecular cloud cores in star-forming regions that could be progenitors of ELFs.

DOI： 10.1088/2041-8205/725/2/L239

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The formation and evolution of the circumstellar disk in unmagnetized molecular clouds is investigated using three-dimensional hydrodynamic simulations from the prestellar core until the end of the main accretion phase. In collapsing cloud cores, the first (adiabatic) core with a size of greater than or similar to 3 AU forms prior to the formation of the protostar. At its formation, the first core has a thick disk-like structure and is mainly supported by the thermal pressure. After the protostar formation, it decreases the thickness gradually and becomes supported by the centrifugal force. We found that the first core is a precursor of the circumstellar disk with a size of >3 AU. This means that unmagnetized protoplanetary disk smaller than <3 AU does not exist. Reflecting the thermodynamics of the collapsing gas, at the protostar formation epoch, the first core (or the circumstellar disk) has a mass of similar to 0.005-0.1 M(circle dot), while the protostar has a mass of similar to 10(-3) M(circle dot). Thus, just after the protostar formation, the circumstellar disk is about 10-100 times more massive than the protostar. In the main accretion phase that lasts for similar to 10(5) yr, the circumstellar disk mass initially tends to dominate the protostellar mass. Such a massive disk is unstable to gravitational instability and tends to show fragmentation. Our calculations indicate that the low-mass companions may form in the circumstellar disk in the main accretion phase. In addition, the mass accretion rate onto the protostar shows a strong time variability that is caused by the torque from the low-mass companions and/or the spiral arms in the circumstellar disk. Such variability provides an important signature for detecting the substellar mass companion in the circumstellar disk around very young protostars.

DOI： 10.1088/0004-637X/724/2/1006

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We use resistive magnetohydrodynamical (MHD) simulations with the nested grid technique to study the formation of protoplanetary disks around protostars from molecular cloud cores that provide the realistic environments for planet formation. We find that gaseous planetary-mass objects are formed in the early evolutionary phase by gravitational instability in regions that are decoupled from the magnetic field and surrounded by the injection points of the MHD outflows during the formation phase of protoplanetary disks. Magnetic decoupling enables massive disks to form and these are subject to gravitational instability, even at similar to 10 AU. The frequent formation of planetary-mass objects in the disk suggests the possibility of constructing a hybrid planet formation scenario, where the rocky planets form later under the influence of the giant planets in the protoplanetary disk.

DOI： 10.1088/2041-8205/718/2/L58

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate gas accretion on to a protoplanet, by considering the thermal effect of gas in three-dimensional hydrodynamical simulations, in which the wide region from a protoplanetary gas disc to a Jovian radius planet is resolved using the nested grid method. We estimate the mass accretion rate and growth time-scale of gas giant planets. The mass accretion rate increases with protoplanet mass for M(p) < M(cri), while it becomes saturated or decreases for M(p) > M(cri), where M(cri) equivalent to 0.036 M(Jup)(a(p)/1 au)0.75, and M(Jup) and a(p) are the Jovian mass and the orbital radius, respectively. This accretion rate is typically two orders of magnitude smaller than that in two-dimensional simulations. The growth time-scale of a gas giant planet or the time-scale of the gas accretion on to the protoplanet is about 105 yr, that is two orders of magnitude shorter than the growth time-scale of the solid core. The thermal effects barely affect the mass accretion rate because the gravitational energy dominates the thermal energy around the protoplanet. The mass accretion rate obtained in our local simulations agrees quantitatively well with those obtained in global simulations with coarser spatial resolution. The mass accretion rate is mainly determined by the protoplanet mass and the property of the protoplanetary disc. We find that the mass accretion rate is correctly calculated when the Hill or Bondi radius is sufficiently resolved. Using the oligarchic growth of protoplanets, we discuss the formation time-scale of gas giant planets.

DOI： 10.1111/j.1365-2966.2010.16527.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We perform a three-dimensional nested-grid radiation magnetohydrodynamics (RMHD) simulation with self-gravity to study the early phase of the low-mass star formation process from a rotating molecular cloud core to a first adiabatic core just before the second collapse begins. Radiation transfer is implemented with the flux-limited diffusion approximation, operator-splitting, and implicit time integrator. In the RMHD simulation, the outer region of the first core attains a higher entropy and its size is larger than that in the magnetohydrodynamics simulations with the barotropic approximation. Bipolar molecular outflow consisting of two components is driven by magnetic Lorentz force via different mechanisms, and shock heating by the outflow is observed. Using the RMHD simulation we can predict and interpret the observed properties of star-forming clouds, first cores, and outflows with millimeter/submillimeter radio interferometers, especially the Atacama Large Millimeter/submillimeter Array.

DOI： 10.1088/2041-8205/714/1/L58

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The fragmentation process in collapsing clouds with various metallicities is studied using three-dimensional nested-grid hydrodynamics. Initial clouds are specified by three parameters: cloud metallicity, initial rotation energy and initial cloud shape. For different combinations of these parameters, we calculate 480 models in total and study cloud evolution, fragmentation conditions, orbital separation and binary frequency. For the cloud to fragment during collapse, the initial angular momentum must be higher than a threshold value, which decreases with decreasing metallicity. Although the exact fragmentation conditions depend also on the initial cloud shape, this dependence is only modest. Our results indicate a higher binary frequency in lower metallicity gas. In particular, with the same median rotation parameter as in the solar neighbourhood, a majority of stars are born as members of binary/multiple systems for < 10-4 Z(circle dot). With initial mass < 0.1 M(circle dot), if fragments are ejected in embryo from the host clouds by multibody interaction, they evolve to substellar-mass objects. This provides a formation channel for low-mass stars in zero- or low-metallicity environments.

DOI： 10.1111/j.1365-2966.2009.15394.x

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We examine emission from a young protostellar object (YPO) with three-dimensional ideal magnetohydrodynamic (MHD) simulations and three-dimensional non-local thermodynamic equilibrium line transfer calculations, and show the first results. To calculate the emission field, we employed a snapshot result of an MHD simulation having young bipolar outflows and a dense protostellar disk (a young circumstellar disk) embedded in an infalling envelope. Synthesized line emission of two molecular species (CO and SiO) shows that subthermally excited SiO lines as a high-density tracer can provide a better probe of the complex velocity field of a YPO, compared to fully thermalized CO lines. In a YPO at the earliest stage when the outflows are still embedded in the collapsing envelope, infall, rotation, and outflow motions have similar speeds. We find that the combined velocity field of these components introduces a great complexity in the line emissions through varying optical thickness and emissivity, such as asymmetric double-horn profiles. We show that the rotation of the outflows, one of the features that characterizes an outflow driven by magneto-centrifugal forces, appears clearly in velocity channel maps and intensity-weighted mean velocity (first moment of velocity) maps. The somewhat irregular morphology of the line emission at this youngest stage is dissimilar to a more evolved object such as young Class 0. High angular resolution observation by, e. g., the Atacama Large Millimeter/submillimeter Array telescope can reveal these features. Our results demonstrate a powerful potential of the synthesized emission of the three-dimensional line transfer to probe the velocity field embedded in the envelope, and further analysis will be able to determine the precise velocity field to assess the dynamics in the YPO to gain a better understanding of star formation.

DOI： 10.1088/0004-637X/703/1/1141

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Brown dwarf formation and star formation efficiency are studied using a nested grid simulation that covers 5 orders of magnitude in spatial scale (10(4)-0.1 AU). Starting with a rotating magnetized compact cloud with a mass of 0.22 M(circle dot) (225 M(Jup)), we follow the cloud evolution until the end of the main accretion phase. An outflow of similar to 5 km s(-1) emerges similar to 100 yr before the protostar formation and does not disappear until the end of the calculation. The mass accretion rate declines from similar to 10(-6) M(circle dot) yr(-1) to similar to 10(-8)-10(-12) M(circle dot) yr(-1) in a short time (similar to 10(4) yr) after the protostar formation. This is because (1) a large fraction of mass is ejected from the host cloud by the protostellar outflow and (2) the gas escapes from the host cloud by the thermal pressure. At the end of the calculation, 74% (167 M(Jup)) of the total mass (225 M(Jup)) is outflowing from the protostar, in which 34% (77 M(Jup)) of the total mass is ejected by the protostellar outflow with supersonic velocity and 40% (90 M(Jup)) escapes with subsonic velocity. On the other hand, 20% (45 M(Jup)) is converted into the protostar and 6% (13 M(Jup)) remains as the circumstellar disk. Thus, the star formation efficiency is epsilon = 0.2. The resultant protostellar mass is in the mass range of brown dwarfs. Our results indicate that brown dwarfs can be formed in compact cores in the same manner as hydrogen-burning stars, and the magnetic field and protostellar outflow are essential in determining the star formation efficiency and stellar mass.

DOI： 10.1088/0004-637X/699/2/L157

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We investigate the accretion of angular momentum onto a protoplanet system using three- dimensional hydrodynamical simulations. We consider a local region around a protoplanet in a protoplanetary disk with sufficiently high spatial resolution. We describe the structure of the gas flow onto and around the protoplanet in detail. We find that the gas flows onto the protoplanet system in the vertical direction, crossing the shock front near the Hill radius of the protoplanet, which is qualitatively different from the picture established by two- dimensional simulations. The specific angular momentum of the gas accreted by the protoplanet system increases with the protoplanet's mass. At a Jovian orbit, when the protoplanet's mass M-p is M-p less than or similar to 1M(Jup), where M-Jup is the Jovian mass, the specific angular momentum increases as j proportional to M-p. On the other hand, it increases as j proportional to M-p(2/3) when the protoplanet's mass is M-p greater than or similar to 1M(Jup). The stronger dependence of the specific angular momentum on the protoplanet's mass for M-p less than or similar to 1M(Jup) is due to the thermal pressure of the gas. The estimated total angular momentum of a system of a gas giant planet and a circumplanetary disk is 2 orders of magnitude larger than those of the present gas giant planets in the solar system. A large fraction of the total angular momentum contributes to the formation of the circumplanetary disk. We also discuss the formation of satellites from the circumplanetary disk.

DOI： 10.1086/590421

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Jet-driving and fragmentation processes in a collapsing primordial cloud are studied using three-dimensional MHD nested grid simulations. Starting from a rotating magnetized spherical cloud with a number density of n(c) similar or equal to 10(3) cm(-3), we follow the evolution of the cloud until the adiabatic core (or protostar) formation epoch, n(c) similar or equal to 10(22) cm(-3). We calculate 36 models parameterizing the initial magnetic gamma(0) and rotational beta(0) energies. The evolution of collapsing primordial clouds is characterized by the ratio of the initial rotational energy to the magnetic energy, gamma(0)/beta(0). The Lorentz force significantly affects cloud evolution when gamma(0) > beta(0), while the centrifugal force dominates the Lorentz force when beta(0) > gamma(0). When the cloud rotates rapidly with an angular velocity of Omega(0) > 10(-17)(n(c)/10(3) cm(-3))(2/3) s(-1) and beta(0) > gamma(0), fragmentation occurs before protostar formation, but no jet appears after protostar formation. On the other hand, when the initial cloud has a magnetic field of B(0) > 10(-9)(n(c)/10(3) cm(-3))(2/3) G and gamma(0) > beta(0), a strong jet appears after protostar formation without fragmentation. Our results indicate that Population III protostars frequently show fragmentation and protostellar jets. Population III stars are therefore born as binary or multiple stellar systems; as in present-day star formation, they can drive strong jets that disturb the interstellar medium significantly, and thus they may induce the formation of next-generation stars.

DOI： 10.1086/591074

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Cloud evolution for various metallicities is investigated by three-dimensional nested grid simulations, in which the initial ratio of rotational to gravitational energy of the host cloud beta(0) (=10(-1) to 10(-6)) and cloud metallicity Z (=0-Z(circle dot)) are parameters. Starting from a central number density of n(c) = 10(4) cm(-3), cloud evolution for 48 models is calculated until the protostar is formed (n(c) similar or equal to 10(23) cm(-3)) or fragmentation occurs. The fragmentation condition depends on both the initial rotational energy and the cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation tends to be suppressed in clouds with higher metallicity. Fragmentation occurs when beta(0) > 10(-3) in clouds with solar metallicity (Z = Z(circle dot)), while fragmentation occurs when beta(0) > 10(-5) in the primordial gas cloud (Z = 0). Clouds with lower metallicity have larger probability of fragmentation, indicating that the binary frequency is a decreasing function of cloud metallicity. Thus, the binary frequency at the early universe (or lower metallicity environment) is higher than at the present day (or higher metallicity environment). In addition, binary stars born from low-metallicity clouds have shorter orbital periods than those from highmetallicity clouds. These trends are explained in terms of the thermal history of the collapsing cloud.

DOI： 10.1086/590109

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We study the effect of poloidal magnetic field on type I planetary migration by linear perturbation analysis in the shearing sheet approximation, and the analytic results are compared with numerical calculations. In contrast to the unmagnetized case, the basic equations that describe the wake due to a planet in the disk allow magnetic resonances at which the density perturbation diverges. In order to simplify the problem, we consider the case without magneto-rotational instability. We perform two sets of analyses, two-dimensional and three-dimensional. In the two-dimensional analysis, we find the generalization of the torque formula previously known in the unmagnetized case. In the three-dimensional calculations, we focus on the disk with very strong magnetic field and derive a new analytic formula for the torque exerted on the planet. We find that when the Alfven velocity is much larger than the sound speed, two-dimensional torque is suppressed and three-dimensional modes dominate, in contrast to the unmagnetized case.

DOI： 10.1086/587027

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The fragmentation process of primordial-gas cores during prestellar collapse is studied using three-dimensional nested-grid hydrodynamics. Starting from the initial central number density of n(c) similar to 10(3) cm(-3), we follow the evolution of rotating spherical cores up to the stellar density n(c) similar or equal to 1022 cm(-3). An initial condition of the cores is specified by three parameters: the ratios of the rotation and thermal energies to the gravitational energy (beta(0) and alpha(0), respectively), and the amplitude of the bar-mode density perturbation (A(phi)). Cores with rotation beta(0) > 10(-6) are found to fragment during the collapse. The fragmentation condition hardly depends on either the initial thermal energy alpha(0) or amplitude of bar- mode perturbation A(phi). Since the critical rotation parameter for fragmentation is lower than that expected in first-star formation, binaries or multiples are also common for the first stars.

DOI： 10.1086/533434

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The driving mechanisms of low- and high-velocity outflows in star formation processes are studied using three-dimensional resistive MHD simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate cloud evolution from the molecular cloud core (n(c) 10(4) cm(-3)) to the stellar core (n(c) = 10(22) cm-3), where n(c) denotes the central density. In the collapsing cloud core, we found two distinct flows: low- velocity flows (similar to 5 km s(-1)) with a wide opening angle, driven from the adiabatic core when the central density exceeds n(c) greater than or similar to 10(12) cm(-3); and high-velocity flows (similar to 30 km s(-1)) with good collimation, driven from the protostar when the central density exceeds n(c) greater than or similar to 10(21) cm(-3). High-velocity flows are enclosed by low- velocity flows after protostar formation. The difference in the degree of collimation between the two flows is caused by the strength of the magnetic field and configuration of the magnetic field lines. The magnetic field around an adiabatic core is strong and has an hourglass configuration; therefore, flows from the adiabatic core are driven mainly by the magnetocentrifugal mechanism and guided by the hourglass-like field lines. In contrast, the magnetic field around the protostar is weak and has a straight configuration owing to ohmic dissipation in the high-density gas region. Therefore, flows from the protostar are driven mainly by the magnetic pressure gradient force and guided by straight field lines. Differing depth of the gravitational potential between the adiabatic core and the protostar causes the difference of flow speed. Low-velocity flows may correspond to the observed molecular outflows, while high-velocity flows may correspond to the observed optical jets. We suggest that the protostellar outflow and the jet are driven by different cores, rather than the outflow being entrained by the jet.

DOI： 10.1086/528364

記述言語：英語   掲載種別：研究論文（学術雑誌）  

Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n = 10(4) cm(-3)) to the stellar core (n similar or equal to 10(22) cm(-3)), where n denotes the central density. We calculated 147 models with different initial magnetic, rotational, and thermal energies and the amplitudes of the nonaxisymmetric perturbation. In a collapsing cloud, fragmentation is mainly controlled by the initial ratio of the rotational to the magnetic energy, regardless of the initial thermal energy and amplitude of the nonaxisymmetric perturbation. The cloud rotation promotes fragmentation, while the magnetic field delays or in some cases suppresses fragmentation through all phases of cloud evolution. The results are categorized into three types. When the clouds have larger rotational energies in relation to magnetic energies, fragmentation occurs in the low-density phase (10(12) cm(-3) less than or similar to n less than or similar to 10(15) cm(-3)) with separations of 3-300 AU. Fragments that appeared in this phase are expected to evolve into wide binary systems. On the other hand, when initial clouds have larger magnetic energies in relation to the rotational energies, fragmentation occurs only in the high-density phase (n greater than or similar to 10(17) cm(-3)) after the clouds experience a significant reduction of the magnetic field owing to the ohmic dissipation. Fragments appearing in this phase have mutual separations of less than or similar to 0.3 AU and are expected to evolve into close binary systems. No fragmentation occurs in the case of sufficiently strong magnetic field, in which single stars are expected to be born. Two types of fragmentation epoch reflect wide and close separations. We might be able to observe a bimodal distribution for the radial separation of the protostar in extremely young stellar groups.

DOI： 10.1086/529133

記述言語：英語   掲載種別：研究論文（学術雑誌）  

The early evolution of the magnetic field and angular momentum of newly formed protostars are studied, using three- dimensional resistive MHD nested grid simulations. Starting with a Bonnor- Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n(c) similar or equal to 10(4) cm(-3) and r = 4.6 x 10(5) AU, where n(c) and r are the central density and radius, respectively) to the stellar core (n(c) less than or similar to 10(12) cm(-3); r similar to 1 R-circle dot). The magnetic field strengths at the centers of clouds with the same initial angular momentum but different magnetic field strengths converge to a certain value as the clouds collapse for n(c) less than or similar to 10(12) cm(-3). For 10(12) cm(-3) less than or similar to n(c) less than or similar to 10(16) cm(-3), ohmic dissipation largely removes the magnetic field from a collapsing cloud core, and the magnetic field lines, which are strongly twisted for n(c) less than or similar to 101(2) cm(-3), are decollimated. The magnetic field lines are twisted and amplified again for n(c) greater than or similar to 10(16) cm(-3), because the magnetic field is recoupled with warm gas. Finally, protostars at their formation epoch (n(c) similar or equal to 10(21) cm(-3)) have magnetic fields of similar to 0.1-1 kG, which is comparable to observations. The magnetic field strength of a protostar depends slightly on the angular momentum of the host cloud. A protostar formed from a slowly rotating cloud core has a stronger magnetic field. The evolution of the angular momentum is closely related to the evolution of the magnetic field. The angular momentum in a collapsing cloud is removed by magnetic effects such as magnetic braking, outflow, and jets. The formed protostars have rotation periods of 0.1-2 days at their formation epoch, which is slightly shorter than observations. This indicates that a further removal mechanism for the angular momentum, such as interactions between the protostar and the disk, wind, or jets, is important in the further evolution of protostars.

DOI： 10.1086/521779

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We study the effect of poloidal magnetic field on type I planetary migration by linear perturbation analysis with the shearing-sheet approximation and the analytic results are compared with numerical calculation. We investigate the cases where magneto-rotational instability (MRI) does not occur: either the disk is two-dimensional, or a very strong field is exerted. We derive formulae for torque exerted on the planet for both cases. We find that two-dimensional torque is suppressed when plasma beta is less than 1 and three-dimensional modes dominate, in contrast to unmagnetized case. © 2008 International Astronomical Union.

DOI： 10.1017/S1743921308016906

記述言語：英語   掲載種別：研究論文（学術雑誌）  

We studied the collapse of rotating molecular cloud cores with inclined magnetic fields, based on three-dimensional numerical simulations. The numerical simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform magnetic field. The magnetic field is initially taken to be inclined from the rotation axis. As the cloud collapses, the magnetic field and rotation axis change their directions. When the rotation is slow and the magnetic field is relatively strong, the direction of the rotation axis changes to align with the magnetic field, as shown earlier by Matsumoto & Tomisaka. When the magnetic field is weak and the rotation is relatively fast, the magnetic field inclines to become perpendicular to the rotation axis. In other words, the evolution of the magnetic field and rotation axis depends on the relative strength of the rotation and magnetic field. Magnetic braking acts to align the rotation axis and magnetic field, while the rotation causes the magnetic field to incline through dynamo action. The latter effect dominates the former when the ratio of the angular velocity to the magnetic field is larger than a critical value Omega(0)/B-0 > 0.39G(1/2)c(s)(-1) where B-0, Omega(0), G, and c(s) denote the initial magnetic field, initial angular velocity, gravitational constant, and sound speed, respectively. When the rotation is relatively strong, the collapsing cloud forms a disk perpendicular to the rotation axis and the magnetic field becomes nearly parallel to the disk surface in the high-density region. A spiral structure appears due to the rotation and the wound up magnetic field in the disk.

DOI： 10.1086/504423

記述言語：英語   掲載種別：研究論文（その他学術会議資料等）  

We investigate self-gravitational collapse of magnetized molecular cloud cores and formation of the outflow. We employ a nested grid in order to resolve fine structures of protostar and outflow generation, of which size is as small as 1 AU, and to follow the whole structure of the molecular cloud core, of which radius reaches 0.1-1 pc, simultaneously. The nested grid allows us to follow the evolution of the cloud core with the high dynamic range of 105-106 in the spatial resolution. In this paper, we introduce implementations of the self-gravitational MHD nested grid code and show applications to early stages in star formation: gravitational collapse of cloud core, "first core" formation, and bipolar outflow ejection. In both cases of single and binary star formation, magnetic fields play important role in the outflow formation. The outflow region has extremely low beta regions of β= 10 -6-10-3, and our code shows no sign of numerical instability even in these low-beta regions. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.cpc.2004.06.033

開催年月日： 2012年9月

記述言語：日本語   会議種別：口頭発表（一般）  

開催地：大分大学   国名：日本国  

初代星の形成・進化を理解することは宇宙の進化を理解する上で重要である。 過去の研究から、一つのダークマターハロー中では単一の大質量初代星 が形成されると考えられてきた。 しかし、過去の研究は球対称を仮定していたために、角運動量の 効果を無視していた。その後の3次元シミュレーションも初代星形成直 後までしか計算していなかったために、初代星形成後の円盤形成段階は 調べられていなかった。 近年、シンクセル法などを用いて複数のグループが初代星形成後の進化 の計算を行った。これらの研究は、原始星誕生後に周囲の円盤が分裂して 複数の星が形成しうることを示した。 しかし、これらの計算では、(i)シンクセルが大きすぎるために分裂が盛ん に起こるであろう円盤の内側領域を計算出来ない、または、 (ii)シンクセルを用いない場合には原始星の内部構造まで計算して しまうために長時間計算が出来ないという問題があった。 この研究では、シンクセルを用いずに原始星形成後の長時間 計算を実現した。具体的には、過去の初代星の進化計算から得られた質量と 半径の関係をガスの状態方程式に組み込んだ。この手法により中心星 は質量の増加と共に半径を増大させる。そのため原始星の内部構造を計算 することは出来ないが、原始星周囲の構造を長時間計算することが可能である。 計算の結果、磁場が無い場合には、他の計算で示されているように 激しく分裂が起こり、多数の星が誕生することが分かった。 いくつかの原始星は中心近傍の比較的質量の大きな星に落下する。また、いくつかの 原始星は中心部から放出される。放出される星の最小質量は木星質量程度で ある。他方、初期宇宙に非常に弱い磁場が存在する場合には、磁気制動によって 角運動量が輸送され星周円盤が成長しない。そのため、古典的 描像のようにダークマターハローの中心に単一の大質量星が誕生する。

開催年月日： 2012年3月

会議種別：口頭発表（一般）  

開催地：龍谷大学   国名：日本国  

星や円盤形成の母体となる分子雲は、磁化されておりマイクロガウス程度 の磁場を保持している。星や円盤は、この磁気星間雲 が重力収縮して形成する。星周円盤は、惑星形成の母体でもあるため、 その形成過程を理解することは星形成のみならず惑星形成の観点からも 重要である。 近年、この重力収縮するガス雲中での円盤の形成について「Magnetic Braking Catastrophe」という困難があると指摘されている。 これは、重力収縮の過程、または星形成後のガス降着段階で、磁場の効果 により角運動量が輸送されてしまい回転円盤が出来ないという問題であ る。 円盤が形成するとその回転によって磁力線が強く捻られる。 すると円盤の角運動量は、この磁力線を捻ることによって星間空間に輸送される。 円盤が回転を続ける限り磁力線が捻られ続け角運動量も輸送され続ける。 そのため、星形成過程では回転円盤が出来ないという主張である。 我々は、磁気星間雲中での星周円盤の形成過程を調べるために、磁気散逸 MHD多層格子法を用いて、星形成前の分子雲コアから星が誕生して円盤が 出来る過程の計算を行った。その結果、分子雲の磁場が強い場合でも回転円盤が 出来ることが分かった。円盤形成の初期段階では、周囲に十分なガスが 残っているために、円盤の角運動量は磁力線を通じて円盤周囲のガス に輸送される。しかし、分子雲が十分進化して、分子雲内のガスが円盤に落下し 円盤ガスよりも軽くなると角運動量を効率的に輸送できなくなる。従って、 分子雲のガスが枯渇しつつある段階では回転円盤の形成が可能となる。 さらに、初期にガス雲が重力的に非常に不安定であり円盤へのガス降着率が 大きい場合には、磁場による角運動量輸送率よりもガス降着によって持ち込 まれる角運動量の方が大きくなり、中心星が非常に若い段階でも十分に大き く重い円盤が出来ることが分かった。

開催年月日： 2011年9月

会議種別：口頭発表（一般）  

開催地：鹿児島大学   国名：日本国  

系外惑星の観測は、主星の重元素量が多い程、惑星が存在する確率が高いこと を示している。 コア・アクリーションシナリオに従うと、このような 重元素量が多い円盤中では固体コアが出来やすくガス惑星も誕生しやす いことが分かる。そのため、主星の重元素量と惑星頻度の関係は、 惑星がコア・アクリーションによって誕生したことの間接的な証拠であると 考えられている。 他方、もう一つの惑星形成シナリオである重力不安定シナリオでは、ダスト や重元素量の違いは、ガス惑星の形成にはほとんど影響を与えないと考えられ ている。しかし、この重力不安定シナリオを考える場合には、円盤の形成も 考慮する必要がある。 この講演では、円盤のサイズや質量がダストの量(重元素量)と密接に関係しており、 コア・アクリーションシナリオと同様に、金属量が高いほど重い円盤が形成し、 重力不安定によりガス惑星形成が誕生しやすいことを示す。 観測されているような分子雲コアを初期条件として、ガスに含まれるダストの量 をパラメータとして、円盤形成の計算を行った。ダストの存在量の違いは、ガス の熱進化とイオン化度を通して磁場の散逸に影響を与える。計算の結果、ダスト の量が多いほど、重くサイズの大きな円盤が出来ることが分かった。これは、 以下の2つの理由による。 (1)ダスト量が多い程、ガスが光学的に厚くなる密度が低く、初期により大きな円盤 を形成する。 (2)ガス中にダストが豊富に存在する程、ガスのイオン化度が低く 磁場がより散逸して磁場制動 による角運動量輸送が非効率的になり大きな円盤を成長させる。 これらの結果は、星形成前の分子雲コアが重元素を豊富に含むほど、重力不安定 によってガス惑星 が誕生しやすいことを示唆している。

開催年月日： 2011年3月

会議種別：口頭発表（一般）  

開催地：筑波大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2011a/

開催年月日： 2010年9月

会議種別：口頭発表（一般）  

開催地：金沢大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2010b/

開催年月日： 2010年3月

会議種別：口頭発表（一般）  

開催地：広島大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2010a/

開催年月日： 2009年9月

会議種別：口頭発表（一般）  

開催地：山口大学吉田キャンパス   国名：日本国  

開催年月日： 2009年3月

会議種別：口頭発表（一般）  

開催地：大阪府立大学中百舌鳥キャンパス   国名：日本国  

開催年月日： 2008年9月

会議種別：口頭発表（一般）  

開催地：岡山理科大学   国名：日本国  

開催年月日： 2008年3月

会議種別：口頭発表（一般）  

開催地：国立オリンピック記念青少年総合センター   国名：日本国  

開催年月日： 2007年9月

会議種別：口頭発表（一般）  

開催地：岐阜大学   国名：日本国  

開催年月日： 2007年3月

会議種別：口頭発表（一般）  

開催地：東海大学   国名：日本国  

開催年月日： 2006年9月

会議種別：口頭発表（一般）  

開催地：九州国際大学   国名：日本国  

開催年月日： 2006年3月

会議種別：口頭発表（一般）  

開催地：和歌山大学   国名：日本国  

開催年月日： 2005年10月

会議種別：口頭発表（一般）  

開催地：北海道大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2005b/

開催年月日： 2005年3月

会議種別：口頭発表（一般）  

開催地：明星大学   国名：日本国  

開催年月日： 2004年9月

会議種別：口頭発表（一般）  

開催地：岩手大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2004b/

開催年月日： 2004年3月

会議種別：口頭発表（一般）  

開催地：名古屋大学   国名：日本国  

開催年月日： 2003年9月

会議種別：口頭発表（一般）  

開催地：愛媛大学   国名：日本国  

開催年月日： 2003年3月

会議種別：口頭発表（一般）  

開催地：東北大学   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2003a/

開催年月日： 2002年10月

会議種別：口頭発表（一般）  

開催地：宮崎シーガイア   国名：日本国  

その他リンク： http://www.asj.or.jp/nenkai/2002b/

開催年月日： 2002年3月

会議種別：口頭発表（一般）  

開催地：茨城大学   国名：日本国  

開催年月日： 2001年10月

会議種別：口頭発表（一般）  

開催地：イーグレ姫路   国名：日本国  

開催年月日： 2001年3月

会議種別：口頭発表（一般）  

開催地：千葉大学   国名：日本国  

開催年月日： 2000年10月

会議種別：口頭発表（一般）  

開催地：群馬県総合教育センター   国名：日本国  

開催年月日： 2018年6月

記述言語：英語  

開催地：Austin   国名：アメリカ合衆国  

The Infrared Doppler (IRD) instrument is a fiber-fed high-resolution NIR spectrometer for the Subaru telescope covering the Y,J,H-bands simultaneously with a maximum spectral resolution of 70,000. The main purpose of IRD is a search for Earth-mass planets around nearby M-dwarfs by precise radial velocity measurements, as well as a spectroscopic characterization of exoplanet atmospheres. We report the current status of the instrument, which is undergoing commissioning at the Subaru Telescope, and the first light observation successfully done in August 2017. The general description of the instrument will be given including spectrometer optics, fiber injection system, cryogenic system, scrambler, and laser frequency comb. A large strategic survey mainly focused on late-type M-dwarfs is planned to start from 2019.

開催年月日： 2017年6月

記述言語：英語  

開催地：Firenze   国名：イタリア共和国  

We study formation and long-term evolution of a circumstellar disk using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues and its radius becomes as large as 200 AUs toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum. Although the spiral arms disappear in a few rotations, new spiral arms form recurrently throughout the Class-0 and I phases as the disk soon becomes unstable again by gas accretion. Using synthetic observation, we compare our model with a recent high-resolution observation of Elias 2-27, whose circumstellar disk has grand design spiral arms, and find good agreement. Our model suggests that the grand design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

開催年月日： 2017年6月

記述言語：英語  

開催地：Firenze   国名：イタリア共和国  

We study formation and long-term evolution of a circumstellar disk using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues and its radius becomes as large as 200 AUs toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum. Although the spiral arms disappear in a few rotations, new spiral arms form recurrently throughout the Class-0 and I phases as the disk soon becomes unstable again by gas accretion. Using synthetic observation, we compare our model with a recent high-resolution observation of Elias 2-27, whose circumstellar disk has grand design spiral arms, and find good agreement. Our model suggests that the grand design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

開催年月日： 2017年6月

記述言語：英語  

開催地：Firenze   国名：イタリア共和国  

We study formation and long-term evolution of a circumstellar disk using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues and its radius becomes as large as 200 AUs toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum. Although the spiral arms disappear in a few rotations, new spiral arms form recurrently throughout the Class-0 and I phases as the disk soon becomes unstable again by gas accretion. Using synthetic observation, we compare our model with a recent high-resolution observation of Elias 2-27, whose circumstellar disk has grand design spiral arms, and find good agreement. Our model suggests that the grand design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

開催年月日： 2017年6月

記述言語：英語  

開催地：Firenze   国名：イタリア共和国  

We study formation and long-term evolution of a circumstellar disk using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues and its radius becomes as large as 200 AUs toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum. Although the spiral arms disappear in a few rotations, new spiral arms form recurrently throughout the Class-0 and I phases as the disk soon becomes unstable again by gas accretion. Using synthetic observation, we compare our model with a recent high-resolution observation of Elias 2-27, whose circumstellar disk has grand design spiral arms, and find good agreement. Our model suggests that the grand design spiral arms around Elias 2-27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

開催年月日： 2015年9月

記述言語：英語  

開催地：Zermatt   国名：スイス連邦  

This review describes realistic evolution of magnetic field and rotation of the protostars, dynamics of outflows and jets, and the formation and evolution of protoplanetary disks. Recent advances in the protostellar collapse simulations cover a huge dynamic range from molecular cloud core density to stellar density in a self-consistent manner and account for all the non-ideal magnetohydrodynamical effects, such as Ohmic resistivity, ambipolar diffusion, and Hall current. We explain the emergence of the first core, i.e., the quasi-hydrostatic object that consists of molecular gas, and the second core, i.e., the protostar. Ohmic dissipation largely removes the magnetic flux from the center of a collapsing cloud core. A fast well-collimated bipolar jet along the rotation axis of the protostar is driven after the magnetic field is re-coupled with warm gas (∼10³ K) around the protostar. The circumstellar disk is born in the "dead zone", a region that is de-coupled from the magnetic field, and the outer radius of the disk increases with that of the dead zone during the early accretion phase. The rapid increase of the disk size occurs after the depletion of the envelope of molecular cloud core. The effect of Hall current may create two distinct populations of protoplanetary disks.

開催年月日： 2015年9月

記述言語：英語  

開催地：Zermatt   国名：スイス連邦  

This review describes realistic evolution of magnetic field and rotation of the protostars, dynamics of outflows and jets, and the formation and evolution of protoplanetary disks. Recent advances in the protostellar collapse simulations cover a huge dynamic range from molecular cloud core density to stellar density in a self-consistent manner and account for all the non-ideal magnetohydrodynamical effects, such as Ohmic resistivity, ambipolar diffusion, and Hall current. We explain the emergence of the first core, i.e., the quasi-hydrostatic object that consists of molecular gas, and the second core, i.e., the protostar. Ohmic dissipation largely removes the magnetic flux from the center of a collapsing cloud core. A fast well-collimated bipolar jet along the rotation axis of the protostar is driven after the magnetic field is re-coupled with warm gas (∼10³ K) around the protostar. The circumstellar disk is born in the "dead zone", a region that is de-coupled from the magnetic field, and the outer radius of the disk increases with that of the dead zone during the early accretion phase. The rapid increase of the disk size occurs after the depletion of the envelope of molecular cloud core. The effect of Hall current may create two distinct populations of protoplanetary disks.

開催年月日： 2015年9月

記述言語：英語  

開催地：Zermatt   国名：スイス連邦  

This review describes realistic evolution of magnetic field and rotation of the protostars, dynamics of outflows and jets, and the formation and evolution of protoplanetary disks. Recent advances in the protostellar collapse simulations cover a huge dynamic range from molecular cloud core density to stellar density in a self-consistent manner and account for all the non-ideal magnetohydrodynamical effects, such as Ohmic resistivity, ambipolar diffusion, and Hall current. We explain the emergence of the first core, i.e., the quasi-hydrostatic object that consists of molecular gas, and the second core, i.e., the protostar. Ohmic dissipation largely removes the magnetic flux from the center of a collapsing cloud core. A fast well-collimated bipolar jet along the rotation axis of the protostar is driven after the magnetic field is re-coupled with warm gas (∼10³ K) around the protostar. The circumstellar disk is born in the "dead zone", a region that is de-coupled from the magnetic field, and the outer radius of the disk increases with that of the dead zone during the early accretion phase. The rapid increase of the disk size occurs after the depletion of the envelope of molecular cloud core. The effect of Hall current may create two distinct populations of protoplanetary disks.

開催年月日： 2015年9月

記述言語：英語  

開催地：Zermatt   国名：スイス連邦  

This review describes realistic evolution of magnetic field and rotation of the protostars, dynamics of outflows and jets, and the formation and evolution of protoplanetary disks. Recent advances in the protostellar collapse simulations cover a huge dynamic range from molecular cloud core density to stellar density in a self-consistent manner and account for all the non-ideal magnetohydrodynamical effects, such as Ohmic resistivity, ambipolar diffusion, and Hall current. We explain the emergence of the first core, i.e., the quasi-hydrostatic object that consists of molecular gas, and the second core, i.e., the protostar. Ohmic dissipation largely removes the magnetic flux from the center of a collapsing cloud core. A fast well-collimated bipolar jet along the rotation axis of the protostar is driven after the magnetic field is re-coupled with warm gas (∼10³ K) around the protostar. The circumstellar disk is born in the "dead zone", a region that is de-coupled from the magnetic field, and the outer radius of the disk increases with that of the dead zone during the early accretion phase. The rapid increase of the disk size occurs after the depletion of the envelope of molecular cloud core. The effect of Hall current may create two distinct populations of protoplanetary disks.

開催年月日： 2014年6月

記述言語：英語  

開催地：Montreal   国名：カナダ  

We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R∼70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.

開催年月日： 2014年6月

記述言語：英語  

開催地：Montreal   国名：カナダ  

We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R∼70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.

開催年月日： 2014年6月

記述言語：英語  

開催地：Montreal   国名：カナダ  

We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R∼70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.

開催年月日： 2014年6月

記述言語：英語  

開催地：Montreal   国名：カナダ  

We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R∼70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.

開催年月日： 2012年7月

記述言語：英語  

開催地：Amsterdam   国名：オランダ王国  

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

開催年月日： 2012年7月

記述言語：英語  

開催地：Amsterdam   国名：オランダ王国  

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

開催年月日： 2012年7月

記述言語：英語  

開催地：Amsterdam   国名：オランダ王国  

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

開催年月日： 2012年7月

記述言語：英語  

開催地：Amsterdam   国名：オランダ王国  

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

We performed MHD simulations of the contraction of rotating, magnetized molecular cloud cores. In the molecular cores, B-field and angular momentum (J) vector are not always aligned. When a first hydrostatic core forms, axisymmetric structure appears and average B and J are parallel in small scale. However, in large scale, the configuration is far from this. This means that contraction process is imprinted on the snapshot. We calculated two mock observations of MHD simulations (1) the polarization of dust thermal emission to reveal the magnetic evolution and (2) the line emissions from interstellar molecules to reveal the evolution of density and velocity. Comparing the mock observations with true ones, we can answer several questions: in which case the hourglass-shaped and S-shaped magnetic fields are seen; how the distribution of polarized intensity is understood; how the first hydrostatic core should be observationally identified.

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

We performed MHD simulations of the contraction of rotating, magnetized molecular cloud cores. In the molecular cores, B-field and angular momentum (J) vector are not always aligned. When a first hydrostatic core forms, axisymmetric structure appears and average B and J are parallel in small scale. However, in large scale, the configuration is far from this. This means that contraction process is imprinted on the snapshot. We calculated two mock observations of MHD simulations (1) the polarization of dust thermal emission to reveal the magnetic evolution and (2) the line emissions from interstellar molecules to reveal the evolution of density and velocity. Comparing the mock observations with true ones, we can answer several questions: in which case the hourglass-shaped and S-shaped magnetic fields are seen; how the distribution of polarized intensity is understood; how the first hydrostatic core should be observationally identified.

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

We performed MHD simulations of the contraction of rotating, magnetized molecular cloud cores. In the molecular cores, B-field and angular momentum (J) vector are not always aligned. When a first hydrostatic core forms, axisymmetric structure appears and average B and J are parallel in small scale. However, in large scale, the configuration is far from this. This means that contraction process is imprinted on the snapshot. We calculated two mock observations of MHD simulations (1) the polarization of dust thermal emission to reveal the magnetic evolution and (2) the line emissions from interstellar molecules to reveal the evolution of density and velocity. Comparing the mock observations with true ones, we can answer several questions: in which case the hourglass-shaped and S-shaped magnetic fields are seen; how the distribution of polarized intensity is understood; how the first hydrostatic core should be observationally identified.

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

We performed MHD simulations of the contraction of rotating, magnetized molecular cloud cores. In the molecular cores, B-field and angular momentum (J) vector are not always aligned. When a first hydrostatic core forms, axisymmetric structure appears and average B and J are parallel in small scale. However, in large scale, the configuration is far from this. This means that contraction process is imprinted on the snapshot. We calculated two mock observations of MHD simulations (1) the polarization of dust thermal emission to reveal the magnetic evolution and (2) the line emissions from interstellar molecules to reveal the evolution of density and velocity. Comparing the mock observations with true ones, we can answer several questions: in which case the hourglass-shaped and S-shaped magnetic fields are seen; how the distribution of polarized intensity is understood; how the first hydrostatic core should be observationally identified.

開催年月日： 2012年6月

記述言語：英語  

開催地：Crete   国名：ギリシャ共和国  

開催年月日： 2010年3月

記述言語：英語  

開催地：Austin, TX   国名：アメリカ合衆国  

The evolution of collapsing primordial clouds and the formation of Population III (Pop III) protostars are investigated with three-dimensional ideal MHD simulations. We follow the collapse of parsec-sized primordial clouds down to the formation of protostars on sub-AU scales. Pop III protostar formation is characterized by the ratio of rotational to magnetic energy of the natal cloud. When the rotational energy is larger than the magnetic energy, fragmentation occurs prior to the formation of the protostar, and a binary or multiple system appears. When the magnetic energy is greater than the rotational energy, a strong (> 100 km s^-1) jet is driven by the circumstellar disk around the protostar, which does not fragment. Thus, even in the early universe, magnetic fields play an important role in the star formation process.

開催年月日： 2010年3月

記述言語：英語  

開催地：Austin, TX   国名：アメリカ合衆国  

The evolution of collapsing primordial clouds and the formation of Population III (Pop III) protostars are investigated with three-dimensional ideal MHD simulations. We follow the collapse of parsec-sized primordial clouds down to the formation of protostars on sub-AU scales. Pop III protostar formation is characterized by the ratio of rotational to magnetic energy of the natal cloud. When the rotational energy is larger than the magnetic energy, fragmentation occurs prior to the formation of the protostar, and a binary or multiple system appears. When the magnetic energy is greater than the rotational energy, a strong (> 100 km s^-1) jet is driven by the circumstellar disk around the protostar, which does not fragment. Thus, even in the early universe, magnetic fields play an important role in the star formation process.

開催年月日： 2010年3月

記述言語：英語  

開催地：Austin, TX   国名：アメリカ合衆国  

The evolution of collapsing primordial clouds and the formation of Population III (Pop III) protostars are investigated with three-dimensional ideal MHD simulations. We follow the collapse of parsec-sized primordial clouds down to the formation of protostars on sub-AU scales. Pop III protostar formation is characterized by the ratio of rotational to magnetic energy of the natal cloud. When the rotational energy is larger than the magnetic energy, fragmentation occurs prior to the formation of the protostar, and a binary or multiple system appears. When the magnetic energy is greater than the rotational energy, a strong (> 100 km s^-1) jet is driven by the circumstellar disk around the protostar, which does not fragment. Thus, even in the early universe, magnetic fields play an important role in the star formation process.

開催年月日： 2010年3月

記述言語：英語  

開催地：Austin, TX   国名：アメリカ合衆国  

The evolution of collapsing primordial clouds and the formation of Population III (Pop III) protostars are investigated with three-dimensional ideal MHD simulations. We follow the collapse of parsec-sized primordial clouds down to the formation of protostars on sub-AU scales. Pop III protostar formation is characterized by the ratio of rotational to magnetic energy of the natal cloud. When the rotational energy is larger than the magnetic energy, fragmentation occurs prior to the formation of the protostar, and a binary or multiple system appears. When the magnetic energy is greater than the rotational energy, a strong (> 100 km s^-1) jet is driven by the circumstellar disk around the protostar, which does not fragment. Thus, even in the early universe, magnetic fields play an important role in the star formation process.

開催年月日： 2003年9月

記述言語：英語  

開催地：Falmouth   国名：アメリカ合衆国  

We investigate self-gravitational collapse of magnetized molecular cloud cores and formation of the outflow. We employ a nested grid in order to resolve fine structures of protostar and outflow generation, of which size is as small as 1 AU, and to follow the whole structure of the molecular cloud core, of which radius reaches 0.1-1 pc, simultaneously. The nested grid allows us to follow the evolution of the cloud core with the high dynamic range of 105-106 in the spatial resolution. In this paper, we introduce implementations of the self-gravitational MHD nested grid code and show applications to early stages in star formation: gravitational collapse of cloud core, "first core" formation, and bipolar outflow ejection. In both cases of single and binary star formation, magnetic fields play important role in the outflow formation. The outflow region has extremely low beta regions of β= 10 ^-6-10^-3, and our code shows no sign of numerical instability even in these low-beta regions.

開催年月日： 2003年9月

記述言語：英語  

開催地：Falmouth   国名：アメリカ合衆国  

We investigate self-gravitational collapse of magnetized molecular cloud cores and formation of the outflow. We employ a nested grid in order to resolve fine structures of protostar and outflow generation, of which size is as small as 1 AU, and to follow the whole structure of the molecular cloud core, of which radius reaches 0.1-1 pc, simultaneously. The nested grid allows us to follow the evolution of the cloud core with the high dynamic range of 105-106 in the spatial resolution. In this paper, we introduce implementations of the self-gravitational MHD nested grid code and show applications to early stages in star formation: gravitational collapse of cloud core, "first core" formation, and bipolar outflow ejection. In both cases of single and binary star formation, magnetic fields play important role in the outflow formation. The outflow region has extremely low beta regions of β= 10 ^-6-10^-3, and our code shows no sign of numerical instability even in these low-beta regions.

開催年月日： 2003年9月

記述言語：英語  

開催地：Falmouth   国名：アメリカ合衆国  

We investigate self-gravitational collapse of magnetized molecular cloud cores and formation of the outflow. We employ a nested grid in order to resolve fine structures of protostar and outflow generation, of which size is as small as 1 AU, and to follow the whole structure of the molecular cloud core, of which radius reaches 0.1-1 pc, simultaneously. The nested grid allows us to follow the evolution of the cloud core with the high dynamic range of 105-106 in the spatial resolution. In this paper, we introduce implementations of the self-gravitational MHD nested grid code and show applications to early stages in star formation: gravitational collapse of cloud core, "first core" formation, and bipolar outflow ejection. In both cases of single and binary star formation, magnetic fields play important role in the outflow formation. The outflow region has extremely low beta regions of β= 10 ^-6-10^-3, and our code shows no sign of numerical instability even in these low-beta regions.

開催年月日： 2003年9月

記述言語：英語  

開催地：Falmouth   国名：アメリカ合衆国  

We investigate self-gravitational collapse of magnetized molecular cloud cores and formation of the outflow. We employ a nested grid in order to resolve fine structures of protostar and outflow generation, of which size is as small as 1 AU, and to follow the whole structure of the molecular cloud core, of which radius reaches 0.1-1 pc, simultaneously. The nested grid allows us to follow the evolution of the cloud core with the high dynamic range of 105-106 in the spatial resolution. In this paper, we introduce implementations of the self-gravitational MHD nested grid code and show applications to early stages in star formation: gravitational collapse of cloud core, "first core" formation, and bipolar outflow ejection. In both cases of single and binary star formation, magnetic fields play important role in the outflow formation. The outflow region has extremely low beta regions of β= 10 ^-6-10^-3, and our code shows no sign of numerical instability even in these low-beta regions.

記述言語：英語  

国名：その他  

In star forming regions, we can observe different evolutionary stages of various objects and phenomena such as molecular clouds, protostellar jets and outflows, circumstellar disks, and protostars. However, it is difficult to directly observe the star formation process itself, because it is veiled by the dense infalling envelope. Numerical simulations can unveil the star formation process in the collapsing gas cloud. Recently, some studies showed protostar formation from the prestellar core stage, in which both molecular clouds and protostars are resolved with sufficient spatial resolution. These simulations showed fragmentation and binary formation, outflow and jet driving, and circumstellar disk formation in the collapsing gas clouds. In addition, the angular momentum transfer and dissipation process of the magnetic field in the star formation process were investigated. In this paper, I review recent developments in numerical simulations of low-mass star formation.

記述言語：英語  

国名：その他  

In star forming regions, we can observe different evolutionary stages of various objects and phenomena such as molecular clouds, protostellar jets and outflows, circumstellar disks, and protostars. However, it is difficult to directly observe the star formation process itself, because it is veiled by the dense infalling envelope. Numerical simulations can unveil the star formation process in the collapsing gas cloud. Recently, some studies showed protostar formation from the prestellar core stage, in which both molecular clouds and protostars are resolved with sufficient spatial resolution. These simulations showed fragmentation and binary formation, outflow and jet driving, and circumstellar disk formation in the collapsing gas clouds. In addition, the angular momentum transfer and dissipation process of the magnetic field in the star formation process were investigated. In this paper, I review recent developments in numerical simulations of low-mass star formation.

記述言語：英語  

国名：その他  

In star forming regions, we can observe different evolutionary stages of various objects and phenomena such as molecular clouds, protostellar jets and outflows, circumstellar disks, and protostars. However, it is difficult to directly observe the star formation process itself, because it is veiled by the dense infalling envelope. Numerical simulations can unveil the star formation process in the collapsing gas cloud. Recently, some studies showed protostar formation from the prestellar core stage, in which both molecular clouds and protostars are resolved with sufficient spatial resolution. These simulations showed fragmentation and binary formation, outflow and jet driving, and circumstellar disk formation in the collapsing gas clouds. In addition, the angular momentum transfer and dissipation process of the magnetic field in the star formation process were investigated. In this paper, I review recent developments in numerical simulations of low-mass star formation.

記述言語：英語  

国名：その他  

In star forming regions, we can observe different evolutionary stages of various objects and phenomena such as molecular clouds, protostellar jets and outflows, circumstellar disks, and protostars. However, it is difficult to directly observe the star formation process itself, because it is veiled by the dense infalling envelope. Numerical simulations can unveil the star formation process in the collapsing gas cloud. Recently, some studies showed protostar formation from the prestellar core stage, in which both molecular clouds and protostars are resolved with sufficient spatial resolution. These simulations showed fragmentation and binary formation, outflow and jet driving, and circumstellar disk formation in the collapsing gas clouds. In addition, the angular momentum transfer and dissipation process of the magnetic field in the star formation process were investigated. In this paper, I review recent developments in numerical simulations of low-mass star formation.

種別：査読等 

外国語雑誌　査読論文数：3

種別：査読等 

外国語雑誌　査読論文数：3

日本語雑誌　査読論文数：0

国際会議録　査読論文数：0

国内会議録　査読論文数：0

種別：査読等 

外国語雑誌　査読論文数：3

日本語雑誌　査読論文数：0

国際会議録　査読論文数：0

国内会議録　査読論文数：0

種別：学会・研究会等