論文 - 須佐 元
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Amplification and Saturation of Turbulent Magnetic Fields in Collapsing Primordial Gas Clouds 国際共著
Higashi, Sho, Susa, Hajime, Federrath, Christoph, Chiaki, Gen
The Astrophysical Journal 2024年2月
Recent numerical studies suggest that magnetic fields play an important role in primordial star formation in the early Universe. However, the detailed evolution of the magnetic field in the collapse phase still has uncertainties because of the complicated physics associated with turbulence in a collapsing magnetized system. Here, we perform a suite of numerical MHD simulations that follow the collapse of magnetized, turbulent primordial gas clouds to investigate the evolution of the magnetic field associated with the turbulence, assuming a polytropic equation of state with exponent γ <SUB>eff</SUB> and with various numerical resolutions. In addition, we generalize the analytic theory of magnetic field growth/saturation so that it can deal with various exponents γ <SUB>eff</SUB> and turbulence energy spectra. We find that the numerical results are well reproduced by the theory for various γ <SUB>eff</SUB> through the collapse phase during the formation of the first stars. The magnetic field is eventually amplified by a factor of 10<SUP>12</SUP>–10<SUP>15</SUP> due to kinematic and nonlinear turbulent dynamo effects and reaches 3%–100% of the equipartition level, depending on γ <SUB>eff</SUB>. We also find that the transition between the kinematic and nonlinear stages can be analytically estimated. These results indicate that the strong magnetic field accompanied by supersonic turbulence is a general property and suggest that it can play a crucial role in the formation of the first stars....
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Merger Conditions of Population III Protostar Binaries
Takanobu Kirihara, Hajime Susa, Takashi Hosokawa, Tomoya Kinugawa
The Astrophysical Journal 950 ( 2 ) 188 - 188 2023年6月
出版者・発行元：American Astronomical Society
Abstract
Massive close binary stars with extremely small separations have been observed, and they are possible progenitors of gravitational-wave sources. The evolution of massive binaries in the protostellar accretion stage is key to understanding their formation process. We, therefore, investigate how close the protostars, consisting of a high-density core and a vast low-density envelope, can approach each other but not coalesce. To investigate the coalescence conditions, we conduct smoothed particle hydrodynamics simulations following the evolution of equal-mass binaries with different initial separations. Since Population (Pop) I and III protostars have similar interior structures, we adopt a specific Pop III model with the mass and radius of 7.75 M<sub>⊙</sub> and 61.1 R<sub>⊙</sub> obtained by the stellar evolution calculations. Our results show that the binary separation decreases due to the transport of the orbital angular momentum to spin angular momentum. If the initial separation is less than about 80% of the sum of the protostellar radius, the binary coalesces in a time shorter than the tidal lock timescale. The mass loss up to the merging is ≲3%. After coalescence, the star rotates rapidly, and its interior structure is independent of the initial separation. We conclude that there must be some orbital shrinking mechanism after the protostars contract to enter the zero-age main-sequence stage.その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/acd1e0/pdf
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Saturation Level of Turbulence in Collapsing Gas Clouds 査読あり
Sho Higashi, Hajime Susa, Gen Chiaki
The Astrophysical Journal 940 ( 1 ) 1 - 10 2022年11月
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Amplification of Turbulence in Contracting Prestellar Cores in Primordial Minihalos 査読あり
Sho Higashi, Hajime Susa, Gen Chiaki
The Astrophysical Journal 915 ( 2 ) 107 - 107 2021年7月
出版者・発行元：American Astronomical Society
We investigate the amplification of turbulence through gravitational
contraction of the primordial gas in minihalos. We perform numerical
simulations to follow the cloud collapse, assuming polytropic equations of
state for different initial turbulent Mach numbers and resolutions. We find
that the turbulent velocity is amplified solely by gravitational contraction,
and eventually becomes comparable to the sound speed, even for small initial
turbulent Mach numbers (${\cal M}_0 \gtrsim 0.05$). We derive an analytic
formula for the amplification of turbulent velocity in a collapsing cloud, and
find that our numerical results are consistent with the formula. These results
suggest that the turbulence can play an important role in collapsing clouds for
general cases.その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ac01c7/pdf
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Merge or Survive: Number of Population III Stars per Minihalo 査読あり
Hajime Susa
The Astrophysical Journal 877 ( 2 ) 99 2019年6月
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Metal-poor star formation triggered by the feedback effects from Pop III stars 査読あり
Gen Chiaki, Hajime Susa, Shingo Hirano
Monthly Notices of the Royal Astronomical Society 475 ( 4 ) 4378 - 4395 2018年2月
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Where are the Low-mass Population III Stars? 査読あり
Ishiyama, Tomoaki; Sudo, Kae; Yokoi, Shingo; Hasegawa, Kenji; Tominaga, Nozomu; Susa, Hajime
The Astrophysical Journal 826 ( 1 ) id.9 2016年7月
共著
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The Mass Spectrum of the First Stars 査読あり
Susa, Hajime; Hasegawa, Kenji; Tominaga, Nozomu
The Astrophysical Journal 792 ( 1 ) 1d.32 2014年9月
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Generation of a Seed Magnetic Field around First Stars: The Biermann Battery Effect 査読あり
Kentaro Doi and Hajime Susa
The Astrophysical Journal 741 ( 2 ) 93 2011年11月
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Smoothed Particle Hydrodynamics coupled with Radiation Transfer
須佐元
Publications of the Astronomical Sciety of Japan 58 ( 445 ) 2006年11月
単著
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Formation of dwarf galaxies during the cosmic reionization
Susa H, Umemura M.
Astrophysical Journal 600 ( 1 I ) 1 - 16 2004年1月
共著
We reanalyze the photoevaporation problem of subgalactic objects irradiated by ultraviolet background (UVB) radiation in a reionized universe. For the purpose, we perform three-dimensional radiation smoothed particle hydrodynamics (RSPH) calculations, in which the radiative transfer is solved by a direct method and the nonequilibrium chemistry of primordial gas including H2 molecules is also incorporated. Attention is concentrated on radiative transfer effects on the UVB for the formation of subgalactic objects with Tvir<~104 K. We consider a reionization model with zreion~7 and also the earlier reionization model (zreion~17) inferred by the Wilkinson Microwave Anisotropy Probe (WMAP). We find that star formation is suppressed appreciably by UVB, but baryons at high-density peaks are self-shielded even during the reionization, forming some amount of stars eventually. In that sense, the photoevaporation for subgalactic systems is not as perfect as argued by one-dimensional spherical calculations. The final stellar fraction depends on the collapse epoch and the mass of the system, almost regardless of the reionization epoch. For instance, a few tenths of the formed stars are born after the cosmic reionization in the zreion~7 model, while more than 90% of the stars are born after the reionization in the WMAP reionization model. Thus, the effects of UVB feedback on the substructure problem with a cold dark matter (CDM) scenario should be evaluated with careful treatment of the radiative transfer. The star clusters formed at high-density peaks coalesce with each other in a dissipationless fashion in a dark matter potential, as a resultant forming a spheroidal system. As a result, these low-mass galaxies have large mass-to-light ratios, such as observed in dwarf spheroidal galaxies in the Local Group....
DOI： 10.1086/379784
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The Thermal Evolution of the Postshock Layer in Pregalactic Clouds
Susa H, Uehara H, Nishi R, Yamada M.
Progress of Theoretical Physics 100 ( 1 ) 63 - 77 1998年7月
共著
We re-examine the thermal evolution of the postshock layer in primordial gas clouds. Comparing the time scales, we find that the evolutionary paths of postshock regions in primordial gas clouds can be basically understood in terms of the diagram drawn in the ionization degree vs temperature plane. The results obtained from the diagram are independent of the density in the case that we do not include photodissociation and photoionization. We also argue that the diagram is not only relevant to the case of the steady postshock flow, but also to the isochorically cooling gas....
DOI： 10.1143/PTP.100.63
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Tanikawa, Ataru, Yoshida, Takashi, Kinugawa, Tomoya, Trani, Alessandro A., Hosokawa, Takashi, Susa, Hajime, Omukai, Kazuyuki
The Astrophysical Journal 926 ( 1 ) 83 - 83 2022年2月
出版者・発行元：American Astronomical Society
<title>Abstract</title>
We investigate the formation of merging binary black holes (BHs) through isolated binary evolution, performing binary population synthesis calculations covering an unprecedentedly wide metallicity range of Population (Pop) I, II, III, and extremely metal-poor (EMP) binary stars. We find that the predicted merger rate density and primary BH mass (<italic>m</italic>
<sub>1</sub>) distribution are consistent with the gravitational wave (GW) observations. Notably, Population III and EMP (<10<sup>−2</sup>
<italic>Z</italic>
<sub>⊙</sub>) binary stars yield most of the pair instability (PI) mass gap events with <italic>m</italic>
<sub>1</sub> = 65–130 <italic>M</italic>
<sub>⊙</sub>. Population III binary stars contribute more to the PI mass gap events with increasing redshift, and all the PI mass gap events have the Population III origin at redshifts ≳8. Our result can be assessed by future GW observations in the following two points. First, there are no binary BHs with <italic>m</italic>
<sub>1</sub> = 100–130 <italic>M</italic>
<sub>⊙</sub> in our result, and thus the <italic>m</italic>
<sub>1</sub> distribution should suddenly drop in the range of <italic>m</italic>
<sub>1</sub> = 100–130 <italic>M</italic>
<sub>⊙</sub>. Second, the PI mass gap event rate should increase toward higher redshift up to ∼11, since those events mainly originate from the Population III binary stars. We find that the following three assumptions are needed to reproduce the current GW observations: a top-heavy stellar initial mass function and the presence of close binary stars for Population III and EMP binary stars, and inefficient convective overshoot in the main-sequence phase of stellar evolution. Without any of the above, the number of PI mass gap events becomes too low to reproduce current GW observations.その他リンク： https://iopscience.iop.org/article/10.3847/1538-4357/ac4247/pdf
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Gravitational Wave Physics and Astronomy in the nascent era 査読あり
Arimoto et al.
Progress of Theoretical and Experimental Physics 2021年4月
DOI： 10.1093/ptep/ptab042
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Ionization degree and magnetic diffusivity in star-forming clouds with different metallicities
Nakauchi, Daisuke, Omukai, Kazuyuki, Susa, Hajime
Monthly Notices of the Royal Astronomical Society 502 ( 3 ) 3394 - 3416 2021年4月
出版者・発行元：OXFORD UNIV PRESS
Magnetic fields play such essential roles in star formation as transporting
angular momentum and driving outflows from a star-forming cloud, thereby
controlling the formation efficiency of a circumstellar disc and also multiple
stellar systems. The coupling of magnetic fields to the gas depends on its
ionization degree. We calculate the temperature evolution and ionization degree
of a cloud for various metallicities of Z/Zsun = 1e-6, 1e-5, 1e-4, 1e-3, 1e-2,
1e-1, and 1. We update the chemical network by reversing all the gas-phase
processes and by considering grain-surface chemistry, including grain
evaporation, thermal ionization of alkali metals, and thermionic emission from
grains. The ionization degree at nH ~ 1e15-1e19 /cm^3 becomes up to eight
orders of magnitude higher than that obtained in the previous model, owing to
the thermionic emission and thermal ionization of K and Na, which have been
neglected so far. Although magnetic fields dissipate owing to ambipolar
diffusion or Ohmic loss at nH < 1e15 /cm^3, the fields recover strong coupling
to the gas at nH ~ 1e15 /cm^3, which is lower by a few orders of magnitude
compared to the previous work. We develop a reduced chemical network by
choosing processes relevant to major coolants and charged species. The reduced
network consists of 104 (161) reactions among 28 (38) species in the absence
(presence, respectively) of ionization sources. The reduced model includes H2
and HD formation on grain surfaces as well as the depletion of O, C, OH, CO,
and H2O on grain surfaces. -
Tanikawa, Ataru, Susa, Hajime, Yoshida, Takashi, Trani, Alessandro A., Kinugawa, Tomoya
The Astrophysical Journal 910 ( 1 ) 30 - 30 2021年3月
出版者・発行元：American Astronomical Society
We present the merger rate density of Population (Pop.) III binary black
holes (BHs) by means of a widely-used binary population synthesis code BSE with
extensions to very massive and extreme metal-poor stars. We consider not only
low-mass BHs (lBHs: $5-50 M_\odot$) but also high-mass BHs (hBHs: $130-200
M_\odot$), where lBHs and hBHs are below and above the pair-instability mass
gap ($50-130 M_\odot$), respectively. Pop. III BH-BHs can be categorized into
three subpopulations: BH-BHs without hBHs (hBH0s: $m_{\rm tot} \lesssim 100
M_\odot$), with one hBH (hBH1s: $m_{\rm tot} \sim 130-260 M_\odot$), and with
two hBHs (hBH2s: $m_{\rm tot} \sim 270-400 M_\odot$), where $m_{\rm tot}$ is
the total mass of a BH-BH. Their merger rate densities at the current universe
are $\sim 0.1$ yr$^{-1}$ Gpc$^{-3}$ for hBH0s, and $\sim 0.01$ yr$^{-1}$
Gpc$^{-3}$ for the sum of hBH1s and hBH2s, provided that the mass density of
Pop. III stars is $\sim 10^{13} M_\odot$ Gpc$^{-3}$. These rates are modestly
insensitive to initial conditions and single star models. The hBH1 and hBH2
mergers can dominate BH-BHs with hBHs discovered in near future. They have low
effective spins $\lesssim 0.2$ in the current universe. The number ratio of the
hBH2s to the hBH1s is high, $\gtrsim 0.1$. We also find BHs in the mass gap (up
to $\sim 85 M_\odot$) merge. These merger rates can be reduced to nearly zero
if Pop. III binaries are always wide ($\gtrsim 100 R_\odot$), and if Pop. III
stars always enter into chemically homogeneous evolution. The presence of close
Pop. III binaries ($\sim 10 R_\odot$) are crucial for avoiding the worst
scenario. -
Early Evolution of Disk, Outflow, and Magnetic Field of Young Stellar Objects: Impact of Dust Model
Tsukamoto, Y., Machida, M. N., Susa, H., Nomura, H., Inutsuka, S.
The Astrophysical Journal 2020年6月
The formation and early evolution of low-mass young stellar objects (YSOs) are investigated using three-dimensional non-ideal magnetohydrodynamics simulations. We investigate the evolution of YSOs up to $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation, at which protostellar mass reaches $\sim 0.1{M}_{\odot }$ . We particularly focus on the impact of the dust model on the evolution. We found that a circumstellar disk is formed in all simulations, regardless of the dust model. Disk size is approximately 10 au at the protostar formation epoch, and it increases to several tens of au at $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation. The disk mass is comparable to the central protostellar mass, and gravitational instability develops. In simulations with small dust sizes, the warp of the pseudodisk develops $\sim {10}^{4}\,\mathrm{yr}$ after protostar formation. The warp strengthens magnetic braking in the disk and decreases disk size. Ion-neutral drift can occur in the infalling envelope when the typical dust size is $a\gtrsim 0.2\,\mu {\rm{m } }$ and the protostar (plus disk) mass is $M\gtrsim 0.1{M}_{\odot }$ . The outflow activity is anticorrelated to the dust size, and the strong outflow appears with small dust grains....
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Driving conditions of protostellar outflows in different star-forming environments 査読あり
Koki Higuchi, Masahiro N Machida, Hajime Susa
Monthly Notices of the Royal Astronomical Society 486 ( 3 ) 3741 - 3754 2019年7月
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Blocking metal accretion onto low-mass population III stars by stellar wind
Tanaka S.J, Chiaki G, Tominaga N, Susa H.
AIP Conference Proceedings 1947 2018年4月
共著
Low-mass population III (PopIII) stars could survive up until the present. They would be found in the halo of Our Galaxy, if they exist. Non-detection of low-mass PopIII stars in our Galaxy suggests that PopIII stars have a top-heavy IMF. On the other hand, some claims that the lack of such stars is as a result of metal enrichment of their surface by accretion of heavy elements from interstellar medium (ISM). We investigate effects of the stellar wind on the metal accretion onto low-mass PopIII stars because accretion of the local ISM onto the Sun is prevented by the solar wind even for neutrals. We find that low-mass PopIII stars traveling across the Galaxy forms the stellar magnetosphere in most of their life. Once the magnetosphere is formed, most of neutral interstellar particles are photoionized before reaching to the stellar surface and are blown away by the wind. This demonstrates that low-mass PopIII stars remain pristine and will be found as metal free stars and that further searches for them are valuable to constrain the IMF of PopIII stars....
DOI： 10.1063/1.5030805
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Evolution of magnetic fields in collapsing star-forming clouds under different environments 査読あり
Koki Higuchi, Masahiro N Machida, Hajime Susa
Monthly Notices of the Royal Astronomical Society 475 ( 3 ) 3331 - 3347 2018年1月