| 町田 正博（まちだ まさひろ） | データ更新日：2024.04.05 |
教授 ／
理学研究院
地球惑星科学部門
太陽惑星系物質科学
| 1. | Ayumu Shoshi, Naoto Harada, Kazuki Tokuda, Yoshihiro Kawasaki, Hayao Yamasaki, Asako Sato, Mitsuki Omura, Masayuki Yamaguchi, Kengo Tachihara, Masahiro N. Machida, Ring Gap Structure around Class I Protostar WL 17, The Astrophysical Journal, 10.3847/1538-4357/ad12b5, 2024.02, 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.. |
| 2. | Mitsuki Omura, Kazuki Tokuda, Masahiro N. Machida, Revealing Multiple Nested Molecular Outflows with Rotating Signatures in HH270mms1-A with ALMA, The Astrophysical Journal, 10.3847/1538-4357/ad19ce, 2024.03. |
| 3. | Shantanu Basu, Mahmoud Sharkawi, Masahiro N. Machida, Outflows Driven from a Magnetic Pseudodisk, The Astrophysical Journal, 10.3847/1538-4357/ad1bf3, 2024.04, 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.. |
| 4. | Yuhua Liu, Satoko Takahashi, Masahiro Machida, Kohji Tomisaka, Josep Miquel Girart, Paul T. P. Ho, Kouichiro Nakanishi, Asako Sato, Dust Polarization of Prestellar and Protostellar Sources in OMC-3, The Astrophysical Journal, 10.3847/1538-4357/ad182d, 2024.03. |
| 5. | Satoko Takahashi, Masahiro N. Machida, Mitsuki Omura, Doug Johnstone, Kazuya Saigo, Naoto Harada, Kohji Tomisaka, Paul T. P. Ho, Luis A. Zapata, Steve Mairs, Gregory J. Herczeg, Kotomi Taniguchi, Yuhua Liu, Asako Sato, An Extremely Young Protostellar Core, MMS 1/OMC-3: Episodic Mass Ejection History Traced by the Micro SiO Jet, The Astrophysical Journal, 10.3847/1538-4357/ad2268, 2024.03. |
| 6. | Asako Sato, Kazuki Tokuda, Masahiro N. Machida, Kengo Tachihara, Naoto Harada, Hayao Yamasaki, Shingo Hirano, Toshikazu Onishi, Yuko Matsushita, Secondary Outflow Driven by the Protostar Ser-emb 15 in Serpens, The Astrophysical Journal, 10.3847/1538-4357/ad0132, 958, 2, 102-102, 2023.11, 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.. |
| 7. | Kazuki Tokuda, Naofumi Fukaya, Kengo Tachihara, Mitsuki Omura, Naoto Harada, Shingo Nozaki, Ayumu Shoshi, Masahiro N. Machida, An ALMA-resolved View of 7000 au Protostellar Gas Ring around the Class I Source CrA-IRS 2 as a Possible Sign of Magnetic Flux Advection, The Astrophysical Journal Letters, 10.3847/2041-8213/acfca9, 2023.10. |
| 8. | Kazuki Tokuda, Naoto Harada, Kei E. I. Tanaka, Tsuyoshi Inoue, Takashi Shimonishi, Yichen Zhang, Marta Sewiło, Yuri Kunitoshi, Ayu Konishi, Yasuo Fukui, Akiko Kawamura, Toshikazu Onishi, Masahiro N. Machida, An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass Protostellar Systems in the Large Magellanic Cloud, The Astrophysical Journal, 10.3847/1538-4357/acefb7, 955, 1, 52-52, 2023.09, 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 104–105.5L⊙ 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.. |
| 9. | Yusuke Tsukamoto, Masahiro N. Machida, Shu-ichiro Inutsuka, Co-evolution of dust grains and protoplanetary disks, PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN, 10.1093/pasj/psad040, 2023.07, 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.. |
| 10. | B. Riaz, W. F. Thi, M. N. Machida, First observations of warm and cold methanol in Class 0/I proto-brown dwarfs, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stad1329, 522, 4, 4934-4954, 2023.07, 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 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.. |
| 11. | Yoshihiro Kawasaki, Masahiro N. Machida, Impact of turbulence intensity and fragmentation velocity on dust particle size evolution and non-ideal magnetohydrodynamics effects, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stad1241, 522, 3, 3679-3692, 2023.07, 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.. |
| 12. | Shingo Hirano, Masahiro N. Machida, Shantanu Basu, Magnetic Effects Promote Supermassive Star Formation in Metal-enriched Atomic-cooling Halos, Astrophysical Journal, 10.3847/1538-4357/acda94, 952, 1, 56-56, 2023.07, 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.. |
| 13. | Taichi Uyama, Charles Beichman, Masayuki Kuzuhara, Markus Janson, Takayuki Kotani, Dimitri Mawet, Bun’ei Sato, Motohide Tamura, Hiroyuki Tako Ishikawa, Bryson Cale, Thayne Currie, Hiroki Harakawa, Thomas Henning, Teruyuki Hirano, Klaus Hodapp, Yasunori Hori, Masato Ishizuka, Shane Jacobson, Yui Kasagi, Eiichiro Kokubo, Mihoko Konishi, Tomoyuki Kudo, Takashi Kurokawa, Nobuhiko Kusakabe, Jungmi Kwon, Masahiro Machida, Takao Nakagawa, Norio Narita, Jun Nishikawa, Masahiro Ogihara, Masashi Omiya, Takuma Serizawa, Akitoshi Ueda, Sébastien Vievard, Ji Wang, Direct Imaging Explorations for Companions around Mid-Late M Stars from the Subaru/IRD Strategic Program, Astronomical Journal, 10.3847/1538-3881/acbf37, 165, 4, 2023.04, 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.. |
| 14. | Naoto Harada, Kazuki Tokuda, Hayao Yamasaki, Asako Sato, Mitsuki Omura, Shingo Hirano, Toshikazu Onishi, Kengo Tachihara, Masahiro N. Machida, Crescent-shaped Molecular Outflow from the Intermediate-mass Protostar DK Cha Revealed by ALMA, The Astrophysical Journal, 10.3847/1538-4357/acb930, 945, 1, 63-63, 2023.03, 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.. |
| 15. | Shingo Nozaki, Masahiro N. Machida, Environmental effects of star-forming cores on mass accretion rate, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stac3819, 519, 4, 5017-5030, 2023.03, 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.. |
| 16. | Asako Sato, Satoko Takahashi, Shun Ishii, Paul T. P. Ho, Masahiro N. Machida, John Carpenter, Luis A. Zapata, Paula Stella Teixeira, Sümeyye Suri, ALMA Fragmented Source Catalog in Orion (FraSCO). I. Outflow Interaction within an Embedded Cluster in OMC-2/FIR 3, FIR 4, and FIR 5, The Astrophysical Journal, 10.3847/1538-4357/aca7c9, 944, 1, 92-92, 2023.02, 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.. |
| 17. | Sakiko Fukaya, Hiroko Shinnaga, Ray S. Furuya, Kohji Tomisaka, Masahiro N. Machida, Naoto Harada, Twisted magnetic field in star formation processes of L1521 F revealed by submillimeter dual band polarimetry using James Clerk Maxwell Telescope, 10.1093/pasj/psac094, 2022.11, 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.. |
| 18. | Yoshihiro Kawasaki, Yoshihiro Kawasaki, Masahiro N. Machida, Implementation of dust particles in three-dimensional magnetohydrodynamics simulation: dust dynamics in a collapsing cloud core, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stac2115, 515, 4, 6073-6092, 2022.10, 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.. |
| 19. | Kazuki Tokuda, Sarolta Zahorecz, Yuri Kunitoshi, Kosuke Higashino, Kei E. I. Tanaka, Ayu Konishi, Taisei Suzuki, Naoya Kitano, Naoto Harada, Takashi Shimonishi, Naslim Neelamkodan, Yasuo Fukui, Akiko Kawamura, Toshikazu Onishi, Masahiro N. Machida, The First Detection of a Protostellar CO Outflow in the Small Magellanic Cloud with ALMA, The Astrophysical Journal Letters, 10.3847/2041-8213/ac81c1, 936, 1, L6-L6, 2022.09, 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⊙.. |
| 20. | Yoshihiro Kawasaki, Shunta Koga, Masahiro N. MacHida, Dust coagulation and fragmentation in a collapsing cloud core and their influence on non-ideal magnetohydrodynamic effects, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stac1919, 515, 2, 2072-2087, 2022.09, 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.. |
| 21. | Gianfranco Bino, Shantanu Basu, Masahiro N. Machida, Aris Tritsis, Mahmoud Sharkawi, Kundan Kadam, Indrani Das, Synthetic Polarization Maps of an Outflow Zone from Magnetohydrodynamic Simulations, Astrophysical Journal, 10.3847/1538-4357/ac7c0f, 936, 1, 2022.09, 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.. |
| 22. | Natsuho Maeda, Keiji Ohtsuki, Takayuki Tanigawa, Masahiro N. Machida, Ryo Suetsugu, Delivery of Gas onto the Circumplanetary Disk of Giant Planets: Planetary-mass Dependence of the Source Region of Accreting Gas and Mass Accretion Rate, The Astrophysical Journal, 10.3847/1538-4357/ac7ddf, 935, 1, 56-56, 2022.08, 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 Mp = 0.05–1MJup at 5.2 au, where MJup 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 Mp ≲ 0.2MJup while it is proportional to Mp for the high-mass regime with Mp ≳ 0.2MJup. 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.. |
| 23. | Shingo Hirano, Masahiro N. Machida, Exponentially Amplified Magnetic Field Eliminates Disk Fragmentation around Population III Protostars, The Astrophysical Journal Letters, 10.3847/2041-8213/ac85e0, 935, 1, L16-L16, 2022.08, 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.. |
| 24. | Hiroyuki Tako Ishikawa, Wako Aoki, Teruyuki Hirano, Takayuki Kotani, Masayuki Kuzuhara, Masashi Omiya, Yasunori Hori, Eiichiro Kokubo, Tomoyuki Kudo, Takashi Kurokawa, Nobuhiko Kusakabe, Norio Narita, Jun Nishikawa, Masahiro Ogihara, Akitoshi Ueda, Thayne Currie, Thomas Henning, Yui Kasagi, Jared R. Kolecki, Jungmi Kwon, Masahiro N. Machida, Michael W. McElwain, Takao Nakagawa, Sebastien Vievard, Ji Wang, Motohide Tamura, Bun'ei Sato, Elemental Abundances of nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey: Proof of Concept, ASTRONOMICAL JOURNAL, 10.3847/1538-3881/ac3ee0, 163, 2, 2022.02, 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 |
| 25. | Tsukamoto, Yusuke, Machida, Masahiro N., Inutsuka, Shu-ichiro, "Ash-fall" induced by molecular outflow in protostar evolution, The Astrophysical Journal Letters, 10.3847/2041-8213/ac2b2f, 2021.09, 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..... |
| 26. | Masahiro Machida, Can High-velocity Protostellar Jets Help to Drive Low-velocity Outflow?, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stab2626, 2021.09, 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 (2πG1/2)-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..... |
| 27. | Harada, Naoto, Hirano, Shingo, Machida, Masahiro N., Hosokawa, Takashi, Impact of Magnetic Braking on High-mass Close Binary Formation, 2021.09, 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 |
| 28. | Chen-Yu Chuang, Yusuke Aso, Naomi Hirano, Shingo Hirano, Masahiro N. Machida, ALMA Observations toward the S-shaped Outflow and the Envelope around NGC 1333 IRAS 4A2, The Astrophysical Journal, 10.3847/1538-4357/abfdbb, 916, 2, 82-82, 2021.08. |
| 29. | Shingo Hirano, Masahiro N. Machida, Shantanu Basu, Supermassive Star Formation in Magnetized Atomic-cooling Gas Clouds: Enhanced Accretion, Intermittent Fragmentation, and Continuous Mergers, The Astrophysical Journal, 10.3847/1538-4357/ac0913, 917, 1, 34-34, 2021.08. |
| 30. | Matsushita, Y., Takahashi, S., Ishii, S., Tomisaka, K., Ho, P.T.P., Carpenter, J.M., Machida, M.N., Super-fast Rotation in the OMC 2/FIR 6b Jet, Astrophysical Journal, 10.3847/1538-4357/ac069f, 916, 1, 2021.07, 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.. |
| 31. | Y. Tsukamoto, M. N. Machida, S. Inutsuka, Conditions for Justifying Single-fluid Approximation for Charged and Neutral Dust Fluids and a Smoothed Particle Magnetohydrodynamics Method for Dust–Gas Mixture, The Astrophysical Journal, 10.3847/1538-4357/abf5db, 913, 2, 2021.06. |
| 32. | Chihomi Hara, Ryohei Kawabe, Fumitaka Nakamura, Naomi Hirano, Shigehisa Takakuwa, Yoshito Shimajiri, Takeshi Kamazaki, James Di Francesco, Masahiro N. Machida, Motohide Tamura, Kazuya Saigo, Tomoaki Matsumoto, Kengo Tomida, Misaligned Twin Molecular Outflows from the Class 0 Protostellar Binary System VLA 1623A Unveiled by ALMA, The Astrophysical Journal, 10.3847/1538-4357/abb810, 912, 1, 34-34, 2021.05. |
| 33. | Kaho Morii, Satoko Takahashi, Masahiro N. Machida, Revealing a Centrally Condensed Structure in OMC-3/MMS 3 with ALMA High-resolution Observations, The Astrophysical Journal, 10.3847/1538-4357/abe61c, 910, 2, 148-148, 2021.04, 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.. |
| 34. | Arimoto, Makoto, Asada, Hideki, Cherry, Michael L., Fujii, Michiko S., Fukazawa, Yasushi, Harada, Akira, Hayama, Kazuhiro, Hosokawa, Takashi, Ioka, Kunihito, Itoh, Yoichi, Kanda, Nobuyuki, Kawabata, Koji S., Kawaguchi, Kyohei, Kawai, Nobuyuki, Kobayashi, Tsutomu, Kohri, Kazunori, Koshio, Yusuke, Kotake, Kei, Kumamoto, Jun, Machida, Masahiro N., Matsufuru, Hideo, Mihara, Tatehiro, Mori, Masaki, Morokuma, Tomoki, Mukohyama, Shinji, Nakano, Hiroyuki, Narikawa, Tatsuya, Negoro, Hitoshi, Nishizawa, Atsushi, Ohgami, Takayuki, Omukai, Kazuyuki, Sakamoto, Takanori, Sako, Shigeyuki, Sasada, Mahito, Sekiguchi, Yuichiro, Serino, Motoko, Soda, Jiro, Sugita, Satoshi, Sumiyoshi, Kohsuke, Susa, Hajime, Suyama, Teruaki, Takahashi, Hirotaka, Takahashi, Kazuya, Takiwaki, Tomoya, Tanaka, Takahiro, Tanaka, Masaomi, Tanikawa, Ataru, Tominaga, Nozomu, Uchikata, Nami, Utsumi, Yousuke, Vagins, Mark R., Yamada, Kei, Yoshida, Michitoshi, Gravitational Wave Physics and Astronomy in the nascent era, 2021.04, 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..... |
| 35. | Yusuke Aso, Masahiro N. Machida, Testing Disk Identification Methods through Numerical Simulations of Protostellar Evolution, The Astrophysical Journal, 10.3847/1538-4357/abc6fc, 2020.12. |
| 36. | Toru Homma, Keiji Ohtsuki, Natsuho Maeda, Ryo Suetsugu, Masahiro N. Machida, Takayuki Tanigawa, Delivery of Pebbles from the Protoplanetary Disk into Circumplanetary Disks, The Astrophysical Journal, 10.3847/1538-4357/abbc08, 903, 2, 98-98, 2020.11. |
| 37. | Kakeru Fujishiro, Kazuki Tokuda, Kengo Tachihara, Tatsuyuki Takashima, Yasuo Fukui, Sarolta Zahorecz, Kazuya Saigo, Tomoaki Matsumoto, Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe André, Akiko Kawamura, Toshikazu Onishi, A Low-velocity Bipolar Outflow from a Deeply Embedded Object in Taurus Revealed by the Atacama Compact Array, The Astrophysical Journal, 10.3847/2041-8213/ab9ca8, 899, 1, L10-L10, 2020.08. |
| 38. | Kazuki Tokuda, Kakeru Fujishiro, Kengo Tachihara, Tatsuyuki Takashima, Yasuo Fukui, Sarolta Zahorecz, Kazuya Saigo, Tomoaki Matsumoto, Kengo Tomida, Masahiro N. Machida, Shu-ichiro Inutsuka, Philippe Andre, Akiko Kawamura, Toshikazu Onishi, FRagmentation and Evolution of Dense Cores Judged by ALMA (FREJA). I. Overview: Inner similar to 1000 au Structures of Prestellar/Protostellar Cores in Taurus, ASTROPHYSICAL JOURNAL, 10.3847/1538-4357/ab9ca7, 899, 1, 2020.08, 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).. |
| 39. | Shingo Hirano, Yusuke Tsukamoto, Shantanu Basu, Masahiro N. Machida, The Effect of Misalignment between the Rotation Axis and Magnetic Field on the Circumstellar Disk, The Astrophysical Journal, 10.3847/1538-4357/ab9f9d, 898, 2, 118-118, 2020.07. |
| 40. | Yu Saiki, Masahiro N. Machida, Twin Jets and Close Binary Formation, ASTROPHYSICAL JOURNAL LETTERS, 10.3847/2041-8213/ab9d86, 897, 2, 2020.07, 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 |
| 41. | Y. Tsukamoto, M. N. Machida, H. Susa, H. Nomura, S. Inutsuka, Early Evolution of Disk, Outflow, and Magnetic Field of Young Stellar Objects: Impact of Dust Model, The Astrophysical Journal, 10.3847/1538-4357/ab93d0, 2020.06. |
| 42. | Tomoya Hirota, Richard L. Plambeck, Melvyn C. H. Wright, Masahiro N. Machida, Yuko Matsushita, Kazuhito Motogi, Mi Kyoung Kim, Ross A. Burns, Mareki Honma, Magnetic Field Structure of Orion Source I, The Astrophysical Journal, 10.3847/1538-4357/ab959e, 2020.06. |
| 43. | Jinshi Sai, Nagayoshi Ohashi, Kazuya Saigo, Tomoaki Matsumoto, Yusuke Aso, Shigehisa Takakuwa, Yuri Aikawa, Ippei Kurose, Hsi Wei Yen, Kohji Tomisaka, Kengo Tomida, Masahiro N. MacHida, Disk Structure around the Class i Protostar L1489 IRS Revealed by ALMA A Warped-disk System, Astrophysical Journal, 10.3847/1538-4357/ab8065, 893, 1, 2020.04, [URL], 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.. |
| 44. | Jinshi Sai, Nagayoshi Ohashi, Kazuya Saigo, Tomoaki Matsumoto, Yusuke Aso, Shigehisa Takakuwa, Yuri Aikawa, Ippei Kurose, Hsi-Wei Yen, Kohji Tomisaka, Kengo Tomida, Masahiro N. Machida, Disk Structure around the Class I Protostar L1489 IRS Revealed by ALMA: A Warped-disk System, ASTROPHYSICAL JOURNAL, 10.3847/1538-4357/ab8065, 893, 1, 2020.04, 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.. |
| 45. | Masahiro N. Machida, Shingo Hirano, Hideyuki Kitta, Misalignment of magnetic fields, outflows, and discs in star-forming clouds, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stz3159, 491, 2, 2180-2197, 2020.01, [URL], 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 |
| 46. | Yusuke Aso, Naomi Hirano, Yuri Aikawa, Masahiro N. Machida, Nagayoshi Ohashi, Masao Saito, Shigehisa Takakuwa, Hsi Wei Yen, Jonathan P. Williams, Protostellar Evolution in Serpens Main Possible Origin of Disk-size Diversity, Astrophysical Journal, 10.3847/1538-4357/ab5284, 887, 2, 2019.12, [URL], 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.. |
| 47. | 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.. |
| 48. | Higuchi Koki, Machida Masahiro N, Susa Hajime, 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, 2019.07. |
| 49. | 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.. |
| 50. | Hirano Shingo, Machida Masahiro N, 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, 2019.06. |
| 51. | 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.. |
| 52. | 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.. |
| 53. | 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 |
| 54. | Yuko Matsushita, Satoko Takahashi, Masahiro N. Machida, Kohji Tomisaka, A Very Compact Extremely High Velocity Flow toward MMS 5/OMC-3 Revealed with ALMA, ASTROPHYSICAL JOURNAL, 10.3847/1538-4357/aaf1b6, 871, 2, 221-221, 2019.02, 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.. |
| 55. | 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.. |
| 56. | 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.. |
| 57. | B. Riaz, M. N. Machida, D. Stamatellos, ALMA reveals a pseudo-disc in a proto-brown dwarf, Monthly Notices of the Royal Astronomical Society, 10.1093/mnras/stz1032, 486, 3, 4114-4129, 2019.01, [URL], 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.. |
| 58. | 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).. |
| 59. | Y. Tsukamoto, S. Okuzumi, K. Iwasaki, M. N. Machida, S. Inutsuka, Does Misalignment between Magnetic Field and Angular Momentum Enhance or Suppress Circumstellar Disk Formation?, The Astrophysical Journal, 10.3847/1538-4357/aae4dc, 868, 1, 22-22, 2018.11. |
| 60. | Ryohei Kawabe, Chihomi Hara, Fumitaka Nakamura, Kazuya Saigo, Takeshi Kamazaki, Yoshito Shimajiri, Kengo Tomida, Shigehisa Takakuwa, Yohko Tsuboi, Masahiro N. Machida, James Di Francesco, Rachel Friesen, Naomi Hirano, Yumiko Oasa, Motohide Tamura, Yoichi Tamura, Takashi Tsukagoshi, David Wilner, Extremely Dense Cores Associated with Chandra Sources in Ophiuchus A Forming Brown Dwarfs Unveiled?, Astrophysical Journal, 10.3847/1538-4357/aae153, 866, 2, 2018.10, [URL], 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 ∼1011-12 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).. |
| 61. | 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.. |
| 62. | 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.. |
| 63. | Higuchi Koki, Machida Masahiro N, Susa Hajime, Evolution of magnetic fields in collapsing star-forming clouds under different environments, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1093/mnras/sty046, 475, 3, 3331-3347, 2018.04. |
| 64. | 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.. |
| 65. | Yuko Matsushita, Yuya Sakurai, Takashi Hosokawa, Masahiro N. 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, 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.. |
| 66. | 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.. |
| 67. | Masahiro N. 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, 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.. |
| 68. | 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 (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.. |
| 69. | 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.. |
| 70. | 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.. |
| 71. | Kazuki Tokuda, Toshikazu Onishi, Kazuya Saigo, Takashi Hosokawa, Tomoaki Matsumoto, Shu-ichiro Inutsuka, Masahiro N. Machida, Kengo Tomida, Masanobu Kunitomo, Akiko Kawamura, Yasuo Fukui, Kengo Tachihara, A Detached Protostellar Disk around a similar to 0.2M(circle dot) 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, 101-101, 2017.11, We report ALMA observations in 0.87 mm continuum and (CO)-C-12 (J = 3-2) toward a very low-luminosity ( |
| 72. | Tomoya Hirota, Masahiro N. Machida, Yuko Matsushita, Kazuhito Motogi, Naoko Matsumoto, Mikyoung Kim, Ross A. Burns, Mareki Honma, ALMA observations of submillimeter H2O and SiO lines in Orion Source I, 2017.11, We present observational results of the submillimeter H2O and SiO lines toward a candidate high-mass young stellar object Orion Source I using ALMA. The spatial structures of the high excitation lines at lower-state energies of >2500 K show compact structures consistent with the circumstellar disk and/or base of the northeast-southwest bipolar outflow with a 100 au scale. The highest excitation transition, the SiO (v=4) line at band 8, has the most compact structure. In contrast, lower-excitation transitions are more extended than 200 au tracing the outflow. Almost all the line show velocity gradients perpendicular to the outflow axis suggesting rotation motions of the circumstellar disk and outflow. While some of the detected lines show broad line profiles and spatially extended emission components indicative of thermal excitation, the strong H2O lines at 321 GHz, 474 GHz, and 658 GHz with brightness temperatures of >1000 K show clear signatures of maser action.. |
| 73. | Yusuke Aso, Nagayoshi Ohashi, Yuri Aikawa, Masahiro N. 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, The Astrophysical Journal, 10.3847/2041-8213/aa9701, 850, 1, 2017.11, 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.. |
| 74. | Yusuke Aso, Nagayoshi Ohashi, Yuri Aikawa, Masahiro N. 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, The Astrophysical Journal, 10.3847/1538-4357/aa8264, 849, 1, 56-56, 2017.10, 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.. |
| 75. | 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.. |
| 76. | Yuko Matsushita, Masahiro N. 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, 2017.08, 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.. |
| 77. | Tomoya Hirota, Masahiro N. 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, 7, 2017.07, 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).. |
| 78. | 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.. |
| 79. | Tomoaki Matsumoto, Masahiro N. Machida, Shu-ichiro Inutsuka, Circumstellar Disks and Outflows in Turbulent Molecular Cloud Cores: Possible Formation Mechanism for Misaligned Systems, The Astrophysical Journal, 10.3847/1538-4357/aa6a1c, 839, 1, 69-69, 2017.04, 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.. |
| 80. | 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.. |
| 81. | Yusuke Aso, Nagayoshi Ohashi, Kazuya Saigo, Shin Koyamatsu, Yuri Aikawa, Masahiko Hayashi, Masahiro N. Machida, Masao Saito, Shigehisa Takakuwa, Kengo Tomida, Kohji Tomisaka, Hsi-Wei Yen, Erratum: “Alma Observations of the Transition from Infall Motion to Keplerian Rotation around the Late-phase Protostar TMC-1A” (2015, ApJ, 812, 27), The Astrophysical Journal, 10.3847/1538-4357/836/1/148, 836, 1, 148-148, 2017.02. |
| 82. | 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.. |
| 83. | 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.. |
| 84. | Kengo Tomida, Masahiro N. 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, 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.. |
| 85. | 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.. |
| 86. | 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.. |
| 87. | Tomoya Hirota, Masahiro N. 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, 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.. |
| 88. | Masahiro N. 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, 2016.12, 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.. |
| 89. | 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.. |
| 90. | Kazuki Tokuda, Toshikazu Onishi, Tomoaki Matsumoto, Kazuya Saigo, Akiko Kawamura, Yasuo Fukui, Shu-ichiro Inutsuka, Masahiro N. Machida, Kengo Tomida, Kengo Tachihara, Philippe Andre, 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, 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.. |
| 91. | 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.. |
| 92. | 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.. |
| 93. | Yusuke Aso, Nagayoshi Ohashi, Kazuya Saigo, Shin Koyamatsu, Yuri Aikawa, Masahiko Hayashi, Masahiro N. 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, 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.. |
| 94. | 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. |
| 95. | 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. |
| 96. | Y. Tsukamoto, K. Iwasaki, S. Okuzumi, M. N. Machida, S. Inutsuka, BIMODALITY OF CIRCUMSTELLAR DISK EVOLUTION INDUCED BY THE HALL CURRENT, The Astrophysical Journal, 10.1088/2041-8205/810/2/L26, 810, 2, 2015.09. |
| 97. | 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.. |
| 98. | Masahiro N. Machida, Teppei Nakamura, 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, The main accretion phase of star formation is investigated in clouds with different metallicities in the range 0 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.. |
| 99. | 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. |
| 100. | Kengo Tomida, Satoshi Okuzumi, 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, 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.. |
| 101. | 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. |
| 102. | 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. |
| 103. | Nagayoshi Ohashi, Kazuya Saigo, Yusuke Aso, Yuri Aikawa, Shin Koyamatsu, Masahiro N. Machida, Masao Saito, Sanemichi Z. Takahashi, Shigehisa Takakuwa, Kengo Tomida, Kohji Tomisaka, Hsi-Wei Yen, 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, 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.. |
| 104. | Masahiro N Machida, PROTOSTELLAR JETS ENCLOSED BY LOW-VELOCITY OUTFLOWS, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/796/1/L17, 796, 1, 2014.11. |
| 105. | Masahiro N. Machida, PROTOSTELLAR JETS ENCLOSED BY LOW-VELOCITY OUTFLOWS, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/796/1/L17, 796, 1, 2014.11, 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.. |
| 106. | 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. |
| 107. | Hsi-Wei Yen, Shigehisa Takakuwa, Nagayoshi Ohashi, Yuri Aikawa, Yusuke Aso, Shin Koyamatsu, Masahiro N. Machida, Kazuya Saigo, Masao Saito, Kengo Tomida, Kohji Tomisaka, 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, 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.. |
| 108. | 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. |
| 109. | Kazuki Tokuda, Toshikazu Onishi, Kazuya Saigo, Akiko Kawamura, Yasuo Fukui, Tomoaki Matsumoto, Shu-ichiro Inutsuka, Masahiro N. Machida, Kengo Tomida, Kengo Tachihara, 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, 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.. |
| 110. | 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.. |
| 111. | Takayuki Tanigawa, Akito Maruta, Masahiro N. Machida, ACCRETION OF SOLID MATERIALS ONTO CIRCUMPLANETARY DISKS FROM PROTOPLANETARY DISKS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/784/2/109, 784, 2, 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)-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.. |
| 112. | 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.. |
| 113. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, 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, 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.. |
| 114. | 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.. |
| 115. | Yusuke Tsukamoto, Masahiro N. Machida, Shu-ichiro Inutsuka, 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, 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.. |
| 116. | 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.. |
| 117. | 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.. |
| 118. | Sanemichi Z. Takahashi, Shu-ichiro Inutsuka, Masahiro N. Machida, A SEMI-ANALYTICAL DESCRIPTION FOR THE FORMATION AND GRAVITATIONAL EVOLUTION OF PROTOPLANETARY DISKS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/770/1/71, 770, 1, 2013.06, 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.. |
| 119. | 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.. |
| 120. | 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.. |
| 121. | 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.. |
| 122. | Yusuke Tsukamoto, Masahiro N. Machida, Formation and early evolution of circumstellar discs in turbulent molecular cloud cores, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1093/mnras/sts111, 428, 2, 1321-1334, 2013.01, 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.. |
| 123. | Kengo Tomida, Kohji Tomisaka, Tomoaki Matsumoto, Yasunori Hori, Satoshi Okuzumi, Masahiro N. Machida, Kazuya Saigo, RADIATION MAGNETOHYDRODYNAMIC SIMULATIONS OF PROTOSTELLAR COLLAPSE: PROTOSTELLAR CORE FORMATION, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/763/1/6, 763, 1, 2013.01, 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.. |
| 124. | 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.. |
| 125. | Akimasa Kataoka, Masahiro N. Machida, Kohji Tomisaka, EXPLORING MAGNETIC FIELD STRUCTURE IN STAR-FORMING CORES WITH POLARIZATION OF THERMAL DUST EMISSION, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/761/1/40, 761, 1, 2012.12, 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.. |
| 126. | 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.. |
| 127. | Hiroko Shinnaga, Giles Novak, John E. Vaillancourt, Masahiro N. Machida, Akimasa Kataoka, Kohji Tomisaka, Jacqueline Davidson, Thomas G. Phillips, C. Darren Dowell, Lerothodi Leeuw, Martin Houde, MAGNETIC FIELD IN THE ISOLATED MASSIVE DENSE CLUMP IRAS 20126+4104, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/750/2/L29, 750, 2, 2012.05, 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.. |
| 128. | 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.. |
| 129. | 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.. |
| 130. | Takayuki Tanigawa, Keiji Ohtsuki, Masahiro N. Machida, DISTRIBUTION OF ACCRETING GAS AND ANGULAR MOMENTUM ONTO CIRCUMPLANETARY DISKS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/747/1/47, 747, 1, 2012.03, 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.. |
| 131. | Masahiro N. Machida, Tomoaki Matsumoto, 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, 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.. |
| 132. | 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.. |
| 133. | Yusuke Tsukamoto, Masahiro N. Machida, 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, 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.. |
| 134. | 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.. |
| 135. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, Effect of Magnetic Braking on Circumstellar Disk Formation in a Strongly Magnetized Cloud, PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN, 10.1093/pasj/63.3.555, 63, 3, 555-573, 2011.06, 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.. |
| 136. | Kengo Tomida, Masahiro N. Machida, Kazuya Saigo, Kohji Tomisaka, Tomoaki Matsumoto, EXPOSED LONG-LIFETIME FIRST CORE: A NEW MODEL OF FIRST CORES BASED ON RADIATION HYDRODYNAMICS (vol 725, L239, 2010), ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/732/1/L18, 732, 1, 2011.05. |
| 137. | 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. |
| 138. | Machida, Masahiro N.; Matsumoto, Tomoaki, The origin and formation of the circumstellar disc, Monthly Notices of the Royal Astronomical Society, 2011.03. |
| 139. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, RECURRENT PLANET FORMATION AND INTERMITTENT PROTOSTELLAR OUTFLOWS INDUCED BY EPISODIC MASS ACCRETION, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/729/1/42, 729, 1, 2011.03, 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.. |
| 140. | 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. |
| 141. | 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. |
| 142. | Kengo Tomida, Masahiro N. Machida, Kazuya Saigo, Kohji Tomisaka, Tomoaki Matsumoto, EXPOSED LONG-LIFETIME FIRST CORE: A NEW MODEL OF FIRST CORES BASED ON RADIATION HYDRODYNAMICS, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/725/2/L239, 725, 2, L239-L244, 2010.12, 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.. |
| 143. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, FORMATION PROCESS OF THE CIRCUMSTELLAR DISK: LONG-TERM SIMULATIONS IN THE MAIN ACCRETION PHASE OF STAR FORMATION, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/724/2/1006, 724, 2, 1006-1020, 2010.12, 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 |
| 144. | 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. |
| 145. | Shu-ichiro Inutsuka, Masahiro N. Machida, Tomoaki Matsumoto, EMERGENCE OF PROTOPLANETARY DISKS AND SUCCESSIVE FORMATION OF GASEOUS PLANETS BY GRAVITATIONAL INSTABILITY, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/718/2/L58, 718, 2, L58-L62, 2010.08, 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.. |
| 146. | 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. |
| 147. | Masahiro N. Machida, Eiichiro Kokubo, Shu-ichiro Inutsuka, Tomoaki Matsumoto, Gas accretion onto a protoplanet and formation of a gas giant planet, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1111/j.1365-2966.2010.16527.x, 405, 2, 1227-1243, 2010.06, 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), 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.. |
| 148. | 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. |
| 149. | Kengo Tomida, Kohji Tomisaka, Tomoaki Matsumoto, Ken Ohsuga, Masahiro N. Machida, Kazuya Saigo, RADIATION MAGNETOHYDRODYNAMICS SIMULATION OF PROTO-STELLAR COLLAPSE: TWO-COMPONENT MOLECULAR OUTFLOW, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/2041-8205/714/1/L58, 714, 1, L58-L63, 2010.05, 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.. |
| 150. | 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. |
| 151. | 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. |
| 152. | Masahiro N. Machida, Kazuyuki Omukai, Tomoaki Matsumoto, Shu-ichiro Inutsuka, Binary formation with different metallicities: dependence on initial conditions, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 10.1111/j.1365-2966.2009.15394.x, 399, 3, 1255-1263, 2009.11, 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 |
| 153. | 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. |
| 154. | 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. |
| 155. | Masako Yamada, Masahiro N. Machida, Shu-ichiro Inutsuka, Kohji Tomisaka, EMISSION FROM A YOUNG PROTOSTELLAR OBJECT. I. SIGNATURES OF YOUNG EMBEDDED OUTFLOWS, ASTROPHYSICAL JOURNAL, 10.1088/0004-637X/703/1/1141, 703, 1, 1141-1158, 2009.09, 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.. |
| 156. | 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. |
| 157. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, FIRST DIRECT SIMULATION OF BROWN DWARF FORMATION IN A COMPACT CLOUD CORE, ASTROPHYSICAL JOURNAL LETTERS, 10.1088/0004-637X/699/2/L157, 699, 2, L157-L160, 2009.07, 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.. |
| 158. | Machida, M. N., Thermal effects of circumplanetary disc formation around proto-gas giant planets, Monthly Notices of the Royal Astronomical Society, 2009.01. |
| 159. | Machida, Masahiro N.; Kokubo, Eiichiro; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Angular Momentum Accretion onto a Gas Giant Planet, The Astrophysical Journal, 2008.10. |
| 160. | Machida, Masahiro N.; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Magnetohydrodynamics of Population III Star Formation, The Astrophysical Journal, 2008.10. |
| 161. | Masahiro N. Machida, Eiichiro Kokubo, Shu-Ichiro Inutsuka, Tomoaki Matsumoto, Angular momentum accretion onto a gas giant planet, ASTROPHYSICAL JOURNAL, 10.1086/590421, 685, 2, 1220-1236, 2008.10, 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.. |
| 162. | Masahiro N. Machida, Tomoaki Matsumoto, Shu-ichiro Inutsuka, Magnetohydrodynamics of population III star formation, ASTROPHYSICAL JOURNAL, 10.1086/591074, 685, 2, 690-704, 2008.10, 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.. |
| 163. | Machida, Masahiro N., Binary Formation in Star-forming Clouds with Various Metallicities, The Astrophysical Journal, 2008.07. |
| 164. | Masahiro N. Machida, Binary formation in star-forming clouds with various metallicities, ASTROPHYSICAL JOURNAL LETTERS, 10.1086/590109, 682, 1, L1-L4, 2008.07, 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.. |
| 165. | 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. |
| 166. | Takayuki Muto, Masahiro N. Machida, Shu-Ichiro Inutsuka, The effect of poloidal magnetic field on type I planetary migration: Significance of magnetic resonance, ASTROPHYSICAL JOURNAL, 10.1086/587027, 679, 1, 813-826, 2008.05, 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.. |
| 167. | 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. |
| 168. | Machida, Masahiro N.; Omukai, Kazuyuki; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Conditions for the Formation of First-Star Binaries, The Astrophysical Journal, 2008.04. |
| 169. | Machida, Masahiro N.; Tomisaka, Kohji; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro, Formation Scenario for Wide and Close Binary Systems, The Astrophysical Journal, 2008.04. |
| 170. | Masahiro N. Machida, Kazuyuki Omukai, Tomoaki Matsumoto, Shu-Ichiro Inutsuka, Conditions for the formation of first-star binaries, ASTROPHYSICAL JOURNAL, 10.1086/533434, 677, 2, 813-827, 2008.04, 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.. |
| 171. | Masahiro N. Machida, Kohji Tomisaka, Tomoaki Matsumoto, Shu-Ichiro Inutsuka, Formation scenario for wide and close binary systems, ASTROPHYSICAL JOURNAL, 10.1086/529133, 677, 1, 327-347, 2008.04, 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.. |
| 172. | Masahiro N. Machida, Shu-ichiro Inutsuka, Tomoaki Matsumoto, High- and low-velocity magnetized outflows in the star formation process in a gravitationally collapsing cloud, ASTROPHYSICAL JOURNAL, 10.1086/528364, 676, 2, 1088-1108, 2008.04, 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.. |
| 173. | Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Magnetic Fields and Rotations of Protostars, The Astrophysical Journal, 2007.12. |
| 174. | Masahiro N. Machida, Shu-Ichiro Inutsuka, Tomoaki Matsumoto, Magnetic fields and rotations of protostars, ASTROPHYSICAL JOURNAL, 10.1086/521779, 670, 2, 1198-1213, 2007.12, 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.. |
| 175. | Takayuki Muto, Masahiro N. MacHida, Shu-Ichiro Inutsuka, The effect of poloidal magnetic field on type i planetary migration, Proceedings of the International Astronomical Union, 10.1017/S1743921308016906, 3, 249, 401-406, 2007.10, 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.. |
| 176. | Machida, Masahiro N.; Inutsuka, Shu-ichiro; Matsumoto, Tomoaki, Outflows Driven by Giant Protoplanets, The Astrophysical Journal, 2006.10. |
| 177. | 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. |
| 178. | 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. |
| 179. | Machida, Masahiro N.; Matsumoto, Tomoaki; Hanawa, Tomoyuki; Tomisaka, Kohji, Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field, The Astrophysical Journal, 2006.07. |
| 180. | Masahiro N. Machida, Tomoaki Matsumoto, Tomoyuki Hanawa, Kohji Tomisaka, Evolution of rotating molecular cloud core with oblique magnetic field, ASTROPHYSICAL JOURNAL, 10.1086/504423, 645, 2, 1227-1245, 2006.07, 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.. |
| 181. | 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. |
| 182. | 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. |
| 183. | 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. |
| 184. | Tomoaki Matsumoto, Masahiro N. Machida, Kohji Tomisaka, Tomoyuki Hanawa, Self-gravitational collapse of a magnetized cloud core: High resolution simulations with three-dimensional MHD nested grid, Computer Physics Communications, 10.1016/j.cpc.2004.06.033, 164, 1-3, 229-236, 2004.12, 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.. |
| 185. | 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. |
| 186. | 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. |
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