九州大学 研究者情報
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町田 正博(まちだ まさひろ) データ更新日:2020.05.06

准教授 /  理学研究院 地球惑星科学部門 太陽惑星系物質科学


主な研究テーマ
ブラックホール連星形成過程の理論的研究
キーワード:ブラックホール
2018.05~2018.03.
ガス惑星形成
キーワード:数値シミュレーション、原始惑星系円盤、惑星
2006.04.
衛星形成
キーワード:衛星、周惑星円盤、ガス惑星
2008.04.
星形成
キーワード:MHD, ジェット、アウトフロー
2003.04.
宇宙初期の星形成、第一世代星形成
キーワード:宇宙論、始原ガス、ファーストスター
2000.04.
研究業績
主要原著論文
1. Koki Higuchi, Masahiro N. Machida, Hajime Susa, Driving conditions of protostellar outflows in different star-forming environments, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stz1079, 486, 3, 3741-3754, 2019.01, [URL], 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 (Cζ = 0, 0.01, 1, and 10, where Cζ is a coefficient controlling the ionization intensity and Cζ = 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 Cζ, 28 different star-forming environments are prepared and simulated. The cloud evolution in each environment is calculated until the central density reaches $n\approx 10^{16}\, {\rm cm}^{-3}$ 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 Cζ ≥ 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..
2. Shingo Hirano, Masahiro N. Machida, Origin of misalignments
Protostellar jet, outflow, circumstellar disc, and magnetic field, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stz740, 485, 4, 4667-4674, 2019.03, [URL], Recent observations uncover various phenomena around the protostar such as misalignment between the outflow and magnetic field, precession of the jet, and time variability of the ejected clumps, whose origins are under debate. We perform a three-dimensional resistive magnetohydrodynamics simulation of the protostar formation in a star-forming core whose rotation axis is tilted at an angle 45. with respect to the initial magnetic field, in which the protostar is resolved with a spatial resolution of 0.01 au. In low-dense outer region, the prestellar core contracts along the magnetic field lines due to the flux freezing. In high-dense inner region, on the other hand, the magnetic dissipation becomes efficient and weakens the magnetic effects when the gas number density exceeds about 1011 cm -3 Then, the normal direction of the flattened disc is aligned with the angular momentum vector. The outflow, jet, and protostellar ejection are driven from different scales of the circumstellar disc and spout in different directions normal to the warped disc. These axes do not coincide with the global magnetic field direction and vary with time. This study demonstrates that a couple of misalignment natures reported by observations can be simultaneously reproduced only by assuming the star-forming core rotating around a different direction from the magnetic field..
3. Masahiro N. Machida, Shantanu Basu, The First Two Thousand Years of Star Formation, Astrophysical Journal, 10.3847/1538-4357/ab18a7, 876, 2, 2019.05, [URL], Starting from a prestellar core with a size of 1.2 ×104 au, we calculate the evolution of a gravitationally collapsing core until ∼2000 yr after protostar formation using a three-dimensional resistive magnetohydrodynamic simulation in which the protostar is resolved with a spatial resolution of 5.6 ×10-3 au. Following protostar formation, a rotationally supported disk is formed. Although the disk size is as small as ∼2-4 au, it remains present until the end of the simulation. Since the magnetic field dissipates and the angular momentum is then not effectively transferred by magnetic effects, the disk surface density gradually increases, and spiral arms develop due to gravitational instability. The disk angular momentum is then transferred mainly by gravitational torques, which induce an episodic mass accretion onto the central protostar. The episodic accretion causes a highly time-variable mass ejection (the high-velocity jet) near the disk inner edge, where the magnetic field is well coupled with the neutral gas. As the mass of the central protostar increases, the jet velocity gradually increases and exceeds ∼100 . The jet opening angle widens with time at its base, while the jet keeps a very good collimation on a large scale. In addition, a low-velocity outflow is driven from the disk outer edge. A cavity-like structure, a bow shock, and several knots, all of which are usually observed in star-forming regions, are produced in the outflowing region..
4. Kazuhito Motogi, Tomoya Hirota, Masahiro Machida, Yoshinori Yonekura, Mareki Honma, Shigehisa Takakuwa, Satoki Matsushita, The First Bird's-eye View of a Gravitationally Unstable Accretion Disk in High-mass Star Formation, Astrophysical Journal Letters, 10.3847/2041-8213/ab212f, 877, 2, 2019.06, [URL], We report on the first bird's-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimeter/submillimeter Array long baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH3OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is ∼5-7 M and accretion rate onto the stellar surface is 3 × 10-3 M yr-1 or higher. The expected stellar age is well younger than 104 yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2-7 M, depending on the dust opacity index. The estimated Toomre's Q parameter is typically 1-2 and can reach 0.4 at the minimum. These Q values clearly satisfy the classical criteria for gravitational instability, and are consistent with recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70% of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar..
5. Kazuki Tokuda, Kengo Tachihara, Kazuya Saigo, Phillipe André, Yosuke Miyamoto, Sarolta Zahorecz, Shu Ichiro Inutsuka, Tomoaki Matsumoto, Tatsuyuki Takashima, Masahiro N. Machida, Kengo Tomida, Kotomi Taniguchi, Yasuo Fukui, Akiko Kawamura, Ken'ichi Tatematsu, Ryo Kandori, Toshikazu Onishi, A centrally concentrated sub-solar-mass starless core in the Taurus L1495 filamentary complex, Publications of the Astronomical Society of Japan, 10.1093/pasj/psz051, 71, 4, 2019.08, [URL], The formation scenario of brown dwarfs is still unclear because observational studies to investigate its initial condition are quite limited. Our systematic survey of nearby low-mass star-forming regions using the Atacama Compact Array (aka the Morita array) and the IRAM 30-m telescope in 1.2 mm continuum has identified a centrally concentrated starless condensation with a central H2 volume density of ∼106 cm-3, MC5-N, connected to a narrow (width ∼0.03 pc) filamentary cloud in the Taurus L1495 region. The mass of the core is ∼ 0.2-0.4, Mo , which is an order of magnitude smaller than typical low-mass pre-stellar cores. Taking into account a typical core to star formation efficiency for pre-stellar cores (∼20%-40%) in nearby molecular clouds, brown dwarf(s) or very low-mass star(s) may be going to be formed in this core. We have found possible substructures at the high-density portion of the core, although much higher angular resolution observation is needed to clearly confirm them. The subsequent N2H+ and N2D+ observations using the Nobeyama 45-m telescope have confirmed the high-deuterium fractionation (∼30%). These dynamically and chemically evolved features indicate that this core is on the verge of proto-brown dwarf or very low-mass star formation and is an ideal source to investigate the initial conditions of such low-mass objects via gravitational collapse and/or fragmentation of the filamentary cloud complex..
6. Satoko Takahashi, Masahiro Machida, Kohji Tomisaka, Paul T.P. Ho, Edward B. Fomalont, Kouichiro Nakanishi, Josep Miquel Girart, ALMA High Angular Resolution Polarization Study
An Extremely Young Class 0 Source, OMC-3/MMS 6, Astrophysical Journal, 10.3847/1538-4357/aaf6ed, 872, 1, 2019.02, [URL], Using the ≈16 km long baseline data obtained with the Atacama Large Millimeter/submillimeter Array (ALMA), we imaged the Stokes I emission and linearly polarized intensity (PI) in the 1.1 mm continuum band of a very young intermediate-mass protostellar source, MMS 6, in the Orion Molecular Cloud-3. The achieved angular resolution, 0.″02 × 0.″03 (≈10 au), shows for the first time a wealth of data on the dust emission polarization in the central 200 au of a protostar. The PI peak is offset to the southeast (SE) by ≈20 au with respect to the Stokes I peak. Its polarization degree is 11% with its E-vector orientation of the position angle ≈135°. A partial ringlike structure with a radius of ≈80 au is detected in PI but not in the Stokes I. Northwest (NW) and SE parts of the ring are bright, with a high polarization degree of 10%, and their E-vector orientations are roughly orthogonal to those observed near the center. We also detected an armlike polarized structure, extending to 1000 au scale to the north, with the E-vectors aligned along the minor axis of the structure. We explored possible origins of the polarized emission by comparing them with magnetohydrodynamical simulations of the toroidal wrapping of the magnetic field. The simulations are consistent with the PI emission in the ringlike and the extended armlike structures observed with ALMA. However, the current simulations do not completely reproduce observed polarization characteristics in the central 50 au. Although the self-scattering model can explain the polarization pattern and positional offset between the Stokes I and PI in the central 50 au, this model is not able to reproduce the observed high degree of polarization..
7. Mi Kyoung Kim, Tomoya Hirota, Masahiro Machida, Yuko Matsushita, Kazuhito Motogi, Naoko Matsumoto, Mareki Honma, Extremely High Excitation SiO Lines in Disk-outflow Systems in Orion Source i, Astrophysical Journal, 10.3847/1538-4357/aafb6b, 872, 1, 2019.02, [URL], We present high-resolution images of the submillimeter SiO line emissions of a massive young stellar object Orion Source I using the Atacama Large Millimeter/submillimeter Array at band 8. We detected the 464 GHz SiO v = 4 J = 11-10 line in Source I, which is the first detection of the SiO v = 4 line in star-forming regions, together with the 465 GHz SiO v = 2 J = 11-10 and the 428 GHz SiO v = 2 J = 10-9 lines with a resolution of 50 au. The SiO v = 2 J = 11-10 and SiO v = 4 J = 11-10 lines have compact structures with a diameter of <80 au. The spatial and velocity distributions suggest that the line emissions are associated with the base of the outflow and the surface of the edge-on disk. In contrast, SiO v = 2 J = 10-9 emission shows a bipolar structure in the direction of northeast-southwest low-velocity outflow with a ∼200 au scale. The emission line exhibits a velocity gradient along the direction of the disk elongation. With the assumption of the ring structure with Keplerian rotation, we estimated the lower limit of the central mass to be 7 and the radius to be 12 au and 26 au..
8. Shunta Koga, Yusuke Tsukamoto, Satoshi Okuzumi, Masahiro Machida, Dependence of Hall coefficient on grain size and cosmic ray rate and implication for circumstellar disc formation, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/sty3524, 484, 2, 2119-2136, 2019.01, [URL], 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..
9. Y. Tsukamoto, S. Okuzumi, K. Iwasaki, Masahiro Machida, S. Inutsuka, Does Misalignment between Magnetic Field and Angular Momentum Enhance or Suppress Circumstellar Disk Formation?, Astrophysical Journal, 10.3847/1538-4357/aae4dc, 868, 1, 2018.11, [URL], 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)..
10. Yusuke Aso, Naomi Hirano, Yuri Aikawa, Masahiro Machida, Shigehisa Takakuwa, Hsi Wei Yen, Jonathan P. Williams, The Distinct Evolutionary Nature of Two Class 0 Protostars in Serpens Main SMM4, Astrophysical Journal, 10.3847/1538-4357/aacf9b, 863, 1, 2018.08, [URL], 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..
11. Kazuki Tokuda, Toshikazu Onishi, Kazuya Saigo, Tomoaki Matsumoto, Tsuyoshi Inoue, Shu Ichiro Inutsuka, Yasuo Fukui, Masahiro Machida, Kengo Tomida, Takashi Hosokawa, Akiko Kawamura, Kengo Tachihara, Warm CO Gas Generated by Possible Turbulent Shocks in a Low-mass Star-forming Dense Core in Taurus, Astrophysical Journal, 10.3847/1538-4357/aac898, 862, 1, 2018.07, [URL], 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..
12. Yuko Matsushita, Yuya Sakurai, Takash Hosokawa, Masahiro Machida, Massive outflows driven by magnetic effects - II. Comparison with observations, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stx3070, 475, 1, 391-403, 2018.03, [URL], 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..
13. Masahiro Machida, Koki Higuchi, Satoshi Okuzumi, Different modes of star formation
Gravitational collapse of magnetically subcritical cloud, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stx2589, 473, 3, 3080-3094, 2018.01, [URL], 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..
14. Kazuki Tokuda, Toshikazu Onishi, Kazuya Saigo, Takashi Hosokawa, Tomoaki Matsumoto, Shu Ichiro Inutsuka, Masahiro Machida, Kengo Tomida, Masanobu Kunitomo, Akiko Kawamura, Yasuo Fukui, Kengo Tachihara, A Detached Protostellar Disk around a ∼0.2 M o Protostar in a Possible Site of a Multiple Star Formation in a Dynamical Environment in Taurus, Astrophysical Journal, 10.3847/1538-4357/aa8e9e, 849, 2, 2017.11, [URL], 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..
15. Yusuke Aso, Nagayoshi Ohashi, Yuri Aikawa, Masahiro Machida, Kazuya Saigo, Masao Saito, Shigehisa Takakuwa, Kengo Tomida, Kohji Tomisaka, Hsi Wei Yen, Jonathan P. Williams, ALMA Observations of SMM11 Reveal an Extremely Young Protostar in Serpens Main Cluster, Astrophysical Journal Letters, 10.3847/2041-8213/aa9701, 850, 1, 2017.11, [URL], 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..
16. Yusuke Aso, Nagayoshi Ohashi, Yuri Aikawa, Masahiro Machida, Kazuya Saigo, Masao Saito, Shigehisa Takakuwa, Kengo Tomida, Kohji Tomisaka, Hsi Wei Yen, ALMA Observations of the Protostar L1527 IRS
Probing Details of the Disk and the Envelope Structures, Astrophysical Journal, 10.3847/1538-4357/aa8264, 849, 1, 2017.11, [URL], 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..
17. Yuko Matsushita, Masahiro Machida, Yuya Sakurai, Takashi Hosokawa, Massive outflows driven by magnetic effects in star-forming clouds with high mass accretion rates, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stx893, 470, 1, 1026-1049, 2017.09, [URL], 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..
18. Tomoaki Matsumoto, Masahiro Machida, Shu Ichiro Inutsuka, Circumstellar Disks and Outflows in Turbulent Molecular Cloud Cores
Possible Formation Mechanism for Misaligned Systems, Astrophysical Journal, 10.3847/1538-4357/aa6a1c, 839, 1, 2017.04, [URL], 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..
19. Tomoya Hirota, Masahiro Machida, Yuko Matsushita, Kazuhito Motogi, Naoko Matsumoto, Mi Kyoung Kim, Ross A. Burns, Mareki Honma, Disk-driven rotating bipolar outflow in Orion Source i, Nature Astronomy, 10.1038/s41550-017-0146, 1, 2017.03, [URL], 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..
20. Kengo Tomida, Masahiro Machida, Takashi Hosokawa, Yuya Sakurai, Chia Hui Lin, Grand-design Spiral Arms in a Young Forming Circumstellar Disk, Astrophysical Journal Letters, 10.3847/2041-8213/835/1/L11, 835, 1, 2017.01, [URL], 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..
21. Yusuke Tsukamoto, Satoshi Okuzumi, Kazunari Iwasaki, Masahiro Machida, Shu Ichiro Inutsuka, The impact of the Hall effect during cloud core collapse
Implications for circumstellar disk evolution, Publications of the Astronomical Society of Japan, 10.1093/pasj/psx113, 69, 6, 2017.01, [URL], 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 Jang 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 Jang, respectively. This suggests that the bimodal evolution of the disk size may occur in the early disk evolutionary phase even if B and Jang 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..
22. Tomoya Hirota, Masahiro Machida, Yuko Matsushita, Kazuhito Motogi, Naoko Matsumoto, Mi Kyoung Kim, Ross A. Burns, Mareki Honma, ALMA BAND 8 CONTINUUM EMISSION from ORION SOURCE i, Astrophysical Journal, 10.3847/1538-4357/833/2/238, 833, 2, 2016.12, [URL], 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..
23. Sanemichi Z. Takahashi, Kengo Tomida, Masahiro Machida, Shu Ichiro Inutsuka, The origin of rotation profiles in star-forming clouds, Monthly Notices of the Royal Astronomical Society, 10.1093/MNRAS/STW1994, 463, 2, 1390-1399, 2016.12, [URL], 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 ∝ r1/2, 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..
24. Kazuki Tokuda, Toshikazu Onishi, Tomoaki Matsumoto, Kazuya Saigo, Akiko Kawamura, Yasuo Fukui, Shu Ichiro Inutsuka, Masahiro Machida, Kengo Tomida, Kengo Tachihara, Philippe André, Revealing a detailed mass distribution of a high-density core MC27/L1521F in Taurus with ALMA, Astrophysical Journal, 10.3847/0004-637X/826/1/26, 826, 1, 2016.07, [URL], 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..
25. Masahiro Machida, Tomoaki Matsumoto, Shu Ichiro Inutsuka, Conditions for circumstellar disc formation - II. Effects of initial cloud stability and mass accretion rate, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stw2256, 463, 4, 4246-4267, 2016.01, [URL], 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..
26. Yusuke Aso, Nagayoshi Ohashi, Kazuya Saigo, Shin Koyamatsu, Yuri Aikawa, Masahiko Hayashi, Masahiro Machida, Masao Saito, Shigehisa Takakuwa, Kengo Tomida, Kohji Tomisaka, Hsi Wei Yen, Alma observations of the transition from infall motion to keplerian rotation around the late-phase protostar tmc-1a, Astrophysical Journal, 10.1088/0004-637X/812/1/27, 812, 1, 2015.10, [URL], 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..
27. Tsukamoto, Y, Iwasaki, K, Okuzumi, S., Masahiro N Machida, Inutsuka, S., Bimodality of Circumstellar Disk Evolution Induced by the Hall Current, The Astrophysical Journal Letters, 10.1088/2041-8205/810/2/L26, 810, 2, 2015.09.
28. Tsukamoto, Y, Iwasaki, K, Okuzumi, S., Masahiro N Machida, Inutsuka, S., Effects of Ohmic and ambipolar diffusion on formation and evolution of first cores, protostars, and circumstellar discs, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stv1290, 452, 1, 2015.09.
29. 町田 正博, 中村 鉄平, Accretion phase of star formation in clouds with different metallicities, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1093/mnras/stu2633, 448, 2, 1405-1429, 2015.04, [URL], 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..
30. Tomida, Kengo, Okuzumi, Satoshi, Masahiro N Machida, RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: NONIDEAL MAGNETOHYDRODYNAMIC EFFECTS AND EARLY FORMATION OF CIRCUMSTELLAR DISKS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/801/2/117, 801, 2, 2015.03.
31. Tsukamoto, Y, Takahashi, S. Z., Masahiro N Machida, Inutsuka, S., Effects of radiative transfer on the structure of self-gravitating discs, their fragmentation and the evolution of the fragments, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1093/mnras/stu2160, 446, 2, 1175-1190, 2015.01.
32. Ohashi, Nagayoshi et al., Masahiro N Machida, FORMATION OF A KEPLERIAN DISK IN THE INFALLING ENVELOPE AROUND L1527 IRS: TRANSFORMATION FROM INFALLING MOTIONS TO KEPLER MOTIONS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/796/2/131, 796, 2, 2014.12.
33. Masahiro N Machida, PROTOSTELLAR JETS ENCLOSED BY LOW-VELOCITY OUTFLOWS, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/796/1/L17, 796, 1, 2014.11.
34. Yen, Hsi-Wei, et al., Masahiro N Machida, ALMA OBSERVATIONS OF INFALLING FLOWS TOWARD THE KEPLERIAN DISK AROUND THE CLASS I PROTOSTAR L1489 IRS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/793/1/1, 793, 1, 2014.09.
35. Tokuda, Kazuki, et al., Masahiro N Machida, ALMA OBSERVATIONS OF A HIGH-DENSITY CORE IN TAURUS: DYNAMICAL GAS INTERACTION AT THE POSSIBLE SITE OF A MULTIPLE STAR FORMATION, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/789/1/L4, 789, 1, 2014.07.
36. Tanigawa, Takayuki, Maruta, Akito, Masahiro N Machida, Accretion of Solid Materials onto Circumplanetary Disks from Protoplanetary Disks, The Astrophysical Journal, 10.1088/0004-637X/784/2/109, 784, 109, 2014.04, 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..
37. Masahiro N Machida, Inutsuka Shu-ichiro, Matsumoto Tomoaki, Conditions for circumstellar disc formation: effects of initial cloud configuration and sink treatment, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stt2343, 438, 3, 2278-2306, 2014.03, [URL], 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..
38. 塚本祐介, 町田 正博, 犬塚修一郎, Formation, orbital and thermal evolution, and survival of planetary-mass clumps in the early phase of circumstellar disc evolution, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stt1684, 436, 2, 1667-1673, 2013.12, [URL], 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..
39. Masahiro N Machida, Kentaro Doi, The formation of Population III stars in gas accretion stage: effects of magnetic fields, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stt1524, 435, 5, 3283-3305, 2013.11, [URL], 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..
40. Sanemichi Z Takahashi, Inutsuka Shu-ichiro, Masahiro N Machida, A Semi-analytical Description for the Formation and Gravitational Evolution of Protoplanetary Disks, The Astrophysical Journal, 10.1088/0004-637X/770/1/71, 770, 1, 71-80, 2013.06, [URL], 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..
41. 町田 正博, 細川隆史, Evolution of protostellar outflow around low-mass protostar, Monthly Notices of the Royal Astronomical Society, 2013.03, [URL], 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..
42. 塚本祐介, 町田 正博, Formation and early evolution of circumstellar discs in turbulent molecular cloud cores, Monthly Notices of the Royal Astronomical Society, 428, 2, 2013.01, [URL], 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..
43. 富田賢吾, 富阪幸治, 町田 正博, Radiation Magnetohydrodynamic Simulations of Protostellar Collapse: Protostellar Core Formation, The Astrophysical Journal, 763, 1, 2013.01, [URL], 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..
44. 片岡 章雅, 町田 正博, 富阪幸治, Exploring Magnetic Field Structure in Star-forming Cores with Polarization of Thermal Dust Emission, The Astrophysical Journal, 761, 1, 2012.12, [URL], 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..
45. Shinnaga, Hiroko; Novak, Giles; Vaillancourt, John E.; Machida, Masahiro N.; Kataoka, Akimasa; Tomisaka, Kohji; Davidson, Jacqueline; Phillips, Thomas G.; Dowell, C. Darren; Leeuw, Lerothodi; Houde, Martin, Magnetic Field in the Isolated Massive Dense Clump IRAS 20126+4104, The Astrophysical Journal Letters, 10.1088/2041-8205/750/2/L29, 750, 2, 2012.05, [URL], 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..
46. Machida, Masahiro N.; Matsumoto, Tomoaki , Impact of protostellar outflow on star formation: effects of the initial cloud mass, Monthly Notices of the Royal Astronomical Society, 10.1111/j.1365-2966.2011.20336.x, 421, 1, 588-607, 2012.03, [URL], 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..
47. Tanigawa, Takayuki; Ohtsuki, Keiji; Machida, Masahiro N., Distribution of Accreting Gas and Angular Momentum onto Circumplanetary Disks, The Astrophysical Journal, Volume, 10.1088/0004-637X/747/1/47, 747, 1, 2012.03, [URL], 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..
48. Tsukamoto, Yusuke; Machida, Masahiro N. , Classification of the circumstellar disc evolution during the main accretion phase, Monthly Notices of the Royal Astronomical Society, 10.1111/j.1365-2966.2011.19081.x, 416, 1, 591-600, 2011.09, [URL], 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..
49. Machida, Masahiro N.; Inutsuka, Shu-Ichiro; Matsumoto, Tomoaki , Effect of Magnetic Braking on Circumstellar Disk Formation in a Strongly Magnetized Cloud, Publications of the Astronomical Society of Japan, 63, 3, 555-573, 2011.06, [URL], 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..
50. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Recurrent Planet Formation and Intermittent Protostellar Outflows Induced by Episodic Mass Accretion, The Astrophysical Journal, 2011.03.
51. Machida, Masahiro N.; Matsumoto, Tomoaki, The origin and formation of the circumstellar disc, Monthly Notices of the Royal Astronomical Society, 2011.03.
52. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Formation Process of the Circumstellar Disk: Long-term Simulations in the Main Accretion Phase of Star Formation, The Astrophysical Journal, 2010.12.
53. Tomida, Kengo; Machida, Masahiro N.; Saigo, Kazuya; Tomisaka, Kohji; Matsumoto, Tomoaki, Exposed Long-lifetime First Core: A New Model of First Cores Based on Radiation Hydrodynamics, The Astrophysical Journal Letters 
, 2010.12.
54. Inutsuka, Shu-ichiro; Machida, Masahiro N.; Matsumoto, Tomoaki, Emergence of Protoplanetary Disks and Successive Formation of Gaseous Planets by Gravitational Instability 
, The Astrophysical Journal Letters, 2010.08.
55. Machida, Masahiro N.; Kokubo, Eiichiro; Inutsuka, Shu-Ichiro; Matsumoto, Tomoaki, Gas accretion onto a protoplanet and formation of a gas giant planet, Monthly Notices of the Royal Astronomical Society, 2010.06.
56. Tomida, Kengo; Tomisaka, Kohji; Matsumoto, Tomoaki; Ohsuga, Ken; Machida, Masahiro N.; Saigo, Kazuya, Radiation Magnetohydrodynamics Simulation of Proto-stellar Collapse: Two-component Molecular Outflow, The Astrophysical Journal Letters, 2010.05.
57. Machida, Masahiro N.; Omukai, Kazuyuki; Matsumoto, Tomoaki, Star Formation in Relic H II Regions of the First Stars: Binarity and Outflow Driving, The Astrophysical Journal, 2009.11.
58. Machida, Masahiro N.; Omukai, Kazuyuki; Matsumoto, Tomoaki; Inutsuka, Shu-Ichiro, Binary formation with different metallicities: dependence on initial conditions, Monthly Notices of the Royal Astronomical Society, 2009.11.
59. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, The Circumbinary Outflow: A Protostellar Outflow Driven by a Circumbinary Disk, The Astrophysical Journal Letters, 2009.10.
60. Yamada, Masako; Machida, Masahiro N.; Inutsuka, Shu-ichiro; Tomisaka, Kohji, Emission from a Young Protostellar Object. I. Signatures of Young Embedded Outflows, The Astrophysical Journal, 2009.09.
61. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, First Direct Simulation of Brown Dwarf Formation in a Compact Cloud Core, The Astrophysical Journal Letters, 2009.07.
62. Machida, M. N., Thermal effects of circumplanetary disc formation around proto-gas giant planets, Monthly Notices of the Royal Astronomical Society, 2009.01.
63. Machida, Masahiro N.; Kokubo, Eiichiro; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Angular Momentum Accretion onto a Gas Giant Planet, The Astrophysical Journal, 2008.10.
64. Machida, Masahiro N.; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Magnetohydrodynamics of Population III Star Formation, The Astrophysical Journal, 2008.10.
65. Machida, Masahiro N., Binary Formation in Star-forming Clouds with Various Metallicities, The Astrophysical Journal, 2008.07.
66. Muto, Takayuki; Machida, Masahiro N.; Inutsuka, Shu-ichiro, The Effect of Poloidal Magnetic Field on Type I Planetary Migration: Significance of Magnetic Resonance, The Astrophysical Journal, 2008.05.
67. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, High- and Low-Velocity Magnetized Outflows in the Star Formation Process in a Gravitationally Collapsing Cloud, The Astrophysical Journal, 2008.04.
68. Machida, Masahiro N.; Omukai, Kazuyuki; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Conditions for the Formation of First-Star Binaries, The Astrophysical Journal, 2008.04.
69. Machida, Masahiro N.; Tomisaka, Kohji; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Formation Scenario for Wide and Close Binary Systems, The Astrophysical Journal, 2008.04.
70. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Magnetic Fields and Rotations of Protostars, The Astrophysical Journal, 2007.12.
71. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Outflows Driven by Giant Protoplanets, The Astrophysical Journal, 2006.10.
72. Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Second Core Formation and High-Speed Jets: Resistive Magnetohydrodynamic Nested Grid Simulations
, The Astrophysical Journal, 2006.08.
73. Machida, Masahiro N.; Omukai, Kazuyuki; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, The First Jets in the Universe: Protostellar Jets from the First Stars, The Astrophysical Journal, 2006.08.
74. Machida, Masahiro N.; Matsumoto, Tomoaki; Hanawa, Tomoyuki; Tomisaka, Kohji, Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field, The Astrophysical Journal, 2006.07.
75. Machida, Masahiro N.; Matsumoto, Tomoaki; Hanawa, Tomoyuki; Tomisaka, Kohji, Collapse and fragmentation of rotating magnetized clouds - II. Binary formation and fragmentation of first cores, Monthly Notices of the Royal Astronomical Society, 2005.09.
76. Machida, Masahiro N.; Matsumoto, Tomoaki; Tomisaka, Kohji; Hanawa, Tomoyuki, Collapse and fragmentation of rotating magnetized clouds - I. Magnetic flux-spin relation, Monthly Notices of the Royal Astronomical Society, 2005.09.
77. Machida, M. N., Tomisaka, K., Nakamura, F., Fujimoto, M. Y., Low-Mass Star Formation Triggered by Supernovae in Primordial Clouds, The Astrophysical Journal, 2005.03.
78. Suda, Takuma; Aikawa, Masayuki; Machida, Masahiro N.; Fujimoto, Masayuki Y.; Iben, Icko, Jr., Is HE 0107-5240 A Primordial Star? The Characteristics of Extremely Metal-Poor Carbon-Rich Stars, The Astrophysical Journal, 2004.08.
79. Machida, Masahiro N.; Tomisaka, Kohji; Matsumoto, Tomoaki, First MHD simulation of collapse and fragmentation of magnetized molecular cloud cores, Monthly Notices of the Royal Astronomical Society, 2004.02.
主要学会発表等
1. 町田 正博, 星磁場と回転の効果による円盤の方向の空間依存性 , 日本天文学会, 2015.09, 近年の観測から星形成領域で大局磁場とジェット、アウトフローの向きが異なることが示唆されている。また、星形成領域の偏光観測によって複雑な形状の磁力線が観測されている。磁場は星の形成に関して重大な役割を果たすが、その振る舞いは単純ではない。収縮するガス雲中でローレンツ力と遠心力によって特定の方向の収縮が抑制させることによって円盤状の構造が出来る。磁場によって作られる円盤状構造をpseudo-diskと呼び、回転によって作られる構造を回転円盤と呼ぶ。.
2. 町田 正博, 星形成初期段階での空間スケールの違いによる角運動量輸送機構, 日本天文学会, 2015.03, 星は分子雲コアが重力収縮して誕生する。観測から分子雲コアの角運動量は原始星の角運動量よりも 4−5 桁程
度大きいことが分かっている。そのため、星が出来る過程でどのようにして角運動量を輸送するかを理解すること
は星形成過程を解明する上で重要である。先行研究によって、星形成過程で現れるアウトフローによって分子雲コ
アが持っていた角運動量の多くの割合が星間空間に微量のガスと共に放出されることが示された。しかし、今ま
での研究では磁場の散逸や原始星形成後に現れるジェットなどによる角運動量輸送過程は調べられていなかった。
この研究では、星形成前の分子雲コアを初期条件として星形成過程を計算を計算した。その際、分子雲コアか
ら原始星のスケールまでを空間分解して原始星形成後 500 年以上の計算を行った。この計算の初期の結果は前回
の天文学会で発表しており、(i) 星周円盤外縁から低速度アウトフローが定常的に現れ、(ii) 原始星近傍からは高
速度ジェットが非定常に出現することが示された。
今回は、星の誕生前後での角運動量輸送機構について発表する。星形成直後、星周円盤の外側では主に磁気制
動によって角運動量が輸送される。他方、円盤の外縁では磁気制動に加えてアウトフローによって角運動量が効率
的に輸送される。円盤の内側では磁場の散逸のために磁場による角運動量輸送は効果的ではない。そのため、こ
の領域では円盤の面密度が上昇し重力不安定性によって非軸対称な構造が発達し、円盤中の非軸対称性によって
生じる重力トルクによる角運動量輸送効率が磁場による輸送効率をはるかに卓越する。また、より小さいスケー
ル (原始星近傍) では、ジェットによる角運動量輸送が効果的となる。さらに、ジェットの非定常性は円盤中での
非軸対構造による非定常降着に起因する。ることが分かった。.
3. 町田 正博, 原始星ジェットの進化, 日本天文学会, 2014.09, 星形成領域の観測から、星形成過程で高速のジェットと低速のアウトフローが現れることが分かっている。し
かし、これらのフローの駆動機構は解明されていない。原始星ジェットは、その速度から重力ポテンシャルの深
い原始星近傍で駆動すると考えられているため、数値シミュレーションによってジェットの駆動を調べるために
は、駆動領域である原始星を分解する必要がある。原始星は 0.01AU 程度の半径を持つ。原始星 (近傍) へは、星
周円盤から質量が供給される。そのため、ガス降着とジェットの関係を調べるためには星周円盤を解像する必要
がある。星周円盤は、1−100AU 程度の大きさを持つ。また、円盤は、分子雲コアから供給されるガスによって成
長する。分子雲コアは、1 万 AU 以上の大きさを持つ。厳密にジェットの駆動を計算するためには、原始星、星周
円盤、分子雲コアを空間分解する必要があるが、これらは空間スケールが 6 桁以上も異なる。また、時間尺度も
大きく異なる。そのため、今までの研究ではジェットの長時間進化を計算することは難しかった。
この研究では、簡単な原始星モデルと多層格子法を用いてジェットの駆動を計算した。星が出来る前の分子雲
コアを初期条件として、中心に原始星が誕生してからおよそ 300 年間ジェットの進化を計算した。結果、ジェット
の駆動は非定常であることが分かった。また、従来考えられていたように、ファーストコアから駆動したアウト
フローの中をジェットが突き進むことにより星形成の初期段階において二種類のフローが現れることが分かった。
計算中、原始星からのジェットは、最大 ∼ 100km/s の速度を持ち 300AU 程度まで達した。また、低速のアウト
フローは ∼ 1km/s の速度を持ち 200AU 程度まで達した。.
4. 町田 正博, 磁気制動と星周円盤の形成を適切に計算するための条件, 日本天文学会, 2013.09, [URL], 星周円盤は星形成の副産物であり惑星形成の母体であるため、その形成や進化 を理解することは重要である。しかし、理論的に星周円盤がいつ、どのように して形成するのかは未だ理解されていない。 従来、星の母体となる分子雲コアの角運動量は、原始星の角運動量よりも遥かに 大きいために 円盤形成は、収縮するガス中での角運動量保存の自然な帰結だと考えられていた。 しかし、近年、磁場の効果により原始星周囲の角運動量が外層に過剰に輸送され てしまい 回転円盤が出来ないという問題が指摘されている(Magnetic Braking Catastrophe)。 円盤形成過程は、磁場の増幅と散逸、また磁気制動とアウトフローによる角運動 量輸送などが 密接に絡み合っているために数値シミュレーションによる理解が必要となる。 しかし、シミュレーションで中心の原始星まで解像してしまうと計算のタイムス テップが 非常に短くなり円盤形成過程を計算することが出来ない。 そのため原始星近傍領域をマスクしてその内側領域を計算しないという手法(シ ンクセル法) が使用されている。 この手法を用いて円盤形成過程を計算した結果、星形成過程(少なくとも ガス降着初期段階)では回転円盤出来ないということが報告がされた(Li et al. 2011)。 しかし、研究者ごとにシンクセルの大きさや結果が大きく異なっている。
この研究では星周円盤を適切に計算するための条件を求めるために、 シンクセルの半径や境界条件などを変えて多くの計算を行った。 先ず、他の研究と同様のシンクセルを採用して過去の研究結果を再現した。 その後、シンク半径を徐々に小さくして計算結果が収束するかどうかを調べた。 その結果、ガス収縮段階において形成するファーストコアを十分な空間解像度で 分解すれば円盤形成過程が適切に計算でき、またClass 0段階の原始星周囲でも 円盤が 形成しうることが分かった。 さらに初期の分子雲コアの状態が星周円盤の形成に与える影響も調べた。.
5. 町田 正博, 原始星の進化とアウトフローの関係, 日本天文学会, 2013.03, [URL], 星形成領域の観測は、星はその誕生時に分子アウトフローというガスの放出現象を 伴うことを示唆している。このアウトフローは、広い開口角を持ち原始星周囲の ガスを 星間空間に放出する。そのため、アウトフローは原始星の進化の過程で重要な役割を 果たすと考えられているが、その駆動機構については未だ論争中である。 従来、アウトフローは、高速でコリメーションの良いジェット成分との相互作用 によっ て駆動されると考えられていた。他方、近年の 分子雲コアから原始星が誕生するまでの数値計算は、アウトフ ローは原始星の周囲に形成する円盤状天体から駆動することを示した。 後者の研究では中心星近傍からのガスの放出現象は示せたが、 長時間計算が困難だったために、計算で得られたフローを観測と比較することは 出来なかった。

この研究では、分子雲コアから原始星の形成を経て、分子雲コアが散逸するまでの 計算を行った。その際、アウトフロー駆動の内側の領域は空間 的に解像せずに分子雲コア全体を計算して、アウトフローの長時間進化を調べた。 また、中心に落下したガスから中心天体への質量降着率を導出して原始星の進化 も同時 に計算し、中心星の進化段階を決定した。 計算の結果、原始星がClass 0段階にある場合には、アウトフローは円盤に降着 するガスから エネルギーを獲得して激しく中心星近傍から放出されるが、原始星がClass I段階に 進化するとアウトフローが新たにエネルギーを獲得することが出来ず徐々に 弱くなることが分かった。 また、計算から得られたアウトフローの質量、運動量、エネルギーの時間進化は 観測と 良く一致した。 さらに、観測から得られている中心星光度とアウトフローの運動量の関係を再現する ことが出来た。 これらの結果は、原始星アウトフローは円盤からの直接駆動されるフローによっ て説明が可能 であることを意味している。
.
6. 町田 正博, 初代星周りの円盤形成と磁場の効果, 日本天文学会, 2012.09, [URL], 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。.
7. 町田正博, 磁気星間雲中での星周円盤の形成, 日本天文学会, 2012.03, 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。.
8. 町田正博、松本倫明, 星周円盤とガス惑星形成における重元素量の影響, 日本天文学会, 2011.09, 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。.
9. 町田正博, 低金属量下での星周円盤の形成, 日本天文学会2011年春季年会, 2011.03, [URL].
10. 町田正博, 星周円盤の起源とその形成過程, 日本天文学会2010年秋季年会, 2010.09, [URL].
11. 町田正博, 原始星アウトフローと星形成率の関係, 日本天文学会2010年春季年会, 2010.03, [URL].
12. 町田正博, 周連星円盤からのアウトフロー, 日本天文学会2009年秋季年会, 2009.09, [URL].
13. 町田正博, 低質量分子雲コア中での褐色矮星の形成可能性, 日本天文学会2009年春季年会, 2009.03, [URL].
14. 町田正博, 巨大ガス惑星と周惑星円盤の形成過程, 日本天文学会2008年秋季年会, 2008.09, [URL].
15. 町田正博, 異なる重元素量を持つガス雲中での星形成過程と連星頻度, 日本天文学会2008年春季年会, 2008.03, [URL].
16. 町田正博, 近接連星と遠隔連星の形成シナリオ, 日本天文学会2007年秋季年会, 2007.09, [URL].
17. 町田正博, 原始惑星系円盤中でのガス惑星の角運動量獲得過程, 日本天文学会2007年春季年会, 2007.03, [URL].
18. 町田正博, 星形成過程における分子流と光学ジェットの駆動メカニズム, 日本天文学会2006年秋季年会, 2006.09, [URL].
19. 町田正博, 磁気散逸MHDによる第二コアからの高速ジェット駆動機構の解明, 日本天文学会2006年春季年会, 2006.03, [URL].
20. 町田正博, 原始巨大惑星へのガス降着とアウトフロー, 日本天文学会2005年秋季年会, 2005.10, [URL].
21. 町田正博, 原始星形成:磁場と回転が平行/垂直になる場合, 日本天文学会2005年春季年会, 2005.03, [URL].
22. 町田正博, 分子雲の分裂条件, 日本天文学会2004年秋季年会, 2004.09, [URL].
23. 町田正博, 原始星の磁束と角運動量, 日本天文学会2004年春季年会, 2004.03, [URL].
24. 町田正博, 分子雲コアの分裂を誘発するディスクとバーの形成, 日本天文学会2003年秋季年会, 2003.09, [URL].
25. 町田正博, 分子雲コアからの連星形成過程とアウトフロー, 日本天文学会2003年春季年会, 2003.03, [URL].
26. 町田正博, 非軸対称揺らぎを入れた星間分子雲の成長過程, 日本天文学会2002年秋季年会, 2002.10, [URL].
27. 町田正博、富阪幸治、松本倫明, 非軸対称揺らぎにおける星間磁気雲の収縮過程, 日本天文学会2002年春季年会, 2002.03, [URL].
28. 町田正博、富阪幸治、松本倫明 , 3次元MHD Nested Grid シミュレーションによる星間磁気雲の重力収縮, 日本天文学会2001年秋季年会, 2001.10, [URL].
29. 町田正博、藤本正行、中村文隆, 初期宇宙における低質量星の形成過程, 日本天文学会2001年春季年会, 2001.03, [URL].
30. 町田正博, SuperNova Triggered Star-Formation in Early Universe, 日本天文学会2000年秋季年会, 2000.10, [URL].
学会活動
所属学会名
日本天文学会
学会誌・雑誌・著書の編集への参加状況
2012.01~2017.06, 天文月報, 国内, 編集委員.
2015.04~2016.03, 天文月報, 国内, 編集委員.
2014.04~2015.03, 天文月報, 国内, 編集委員.
2013.01~2016.12, 天文月報, 国内, 編集委員.
学術論文等の審査
年度 外国語雑誌査読論文数 日本語雑誌査読論文数 国際会議録査読論文数 国内会議録査読論文数 合計
2019年度      
2018年度      
2016年度
2017年度
2015年度      
2014年度      
2013年度      
2012年度      
2011年度      
2010年度      
2009年度      
2008年度      
その他の研究活動
海外渡航状況, 海外での教育研究歴
Western Ontario University, Canada, 2019.04~2019.09.
Western Ontario University, Canada, 2018.08~2018.09.
受賞
日本天文学会欧文研究報告論文賞, 日本天文学会, 2017.03.
平成28年度 HPCI 優秀成果賞, High Performance Computing Infrastructure, 2017.10.
平成24年度 科学技術分野の文部科学大臣表彰 若手科学賞, 文部科学省, 2012.04.
研究資金
科学研究費補助金の採択状況(文部科学省、日本学術振興会)
2017年度~2021年度, 基盤研究(B), 分担, 極初期宇宙における星形成の研究.
2017年度~2022年度, 新学術領域研究, 分担, ブラックホール連星形成過程の理論的研究.
2013年度~2017年度, 基盤研究(C), 代表, 新しい星形成シナリオの構築.
2009年度~2011年度, 若手研究(B), 代表, 原始星アウトフローの長時間進化と星周円盤の形成過程.
2006年度~2008年度, 若手研究(B), 代表, 星間分子雲の収縮から第二コアの形成と高速ジェット現象の解明.

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