論文 - 須佐 元
-
Radiative feedback of POPIII stars
須佐 元
HI Survival Through Cosmic Times 2007年6月
-
Photodissociation feedback of population III stars onto neighboring prestellar cores
Susa H.
Astrophysical Journal 659 ( 2 I ) 908 - 917 2007年4月
単著
We investigate the star formation process in the primordial environment in the presence of radiative feedback from other Population III stars that formed earlier. In this paper, we focus our attention on the effects of photodissociative radiation, leading toward a full understanding of the radiative feedback effects. We perform three-dimensional radiation hydrodynamics simulations on this issue, as well as analytic estimates, paying special attention to the self-shielding effect and the dynamics of the star-forming cloud. As a result, we find that the ignition timing of the source star is crucial. If the ignition is later than the epoch when the central density of the collapsing cloud exceeds ~103-104 cm-3, the collapse cannot be reversed, even if the source star is located at <~100 pc. The uncertainty of the critical density comes from the variety of initial conditions of the collapsing cloud. We also find the analytic criterion for a cloud to collapse with a given central density, temperature, and Lyman-Werner-band flux that irradiates the cloud. Although we focus on the radiation from neighboring stars, this result can also be applied to the effects of the diffuse Lyman-Werner (LW) radiation field that is expected to be built up prior to the reionization of the universe. We find that self-gravitating clouds can easily self-shield from diffuse LW radiation and continue their collapse for densities larger than ~103 cm-3....
DOI： 10.1086/512670
-
Maki H, Susa H.
Publications of the Astronomical Society of Japan 59 ( 4 ) 786 - 797 2007年
-
Cosmological Radiative Transfer Codes Comparison Project I: The Static Density Field Tests
須佐元
Monthly Notice of the Royal Astronimical Society 371 ( 1057 ) 2006年11月
単著
-
Secondary Star Formation in a Population III Object
須佐元
Astrophysical Journal Volume 645 ( L93 ) 2006年11月
単著
-
Cosmological radiative transfer codes comparison project - I. The static density field tests
Iliev I.T, Ciardi B, Alvarez M.A, Maselli A, Ferrara A, Gnedin N.Y, Mellema G, Nakamoto T, Norman M.L, Razoumov A.O, Rijkhorst E.-J, Ritzerveld J, Shapiro P.R, Susa H, Umemura M, Whalen D.J.
Monthly Notices of the Royal Astronomical Society 371 ( 3 ) 1057 - 1086 2006年9月
共著
Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal HII region expansion; (2) HII region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form....
-
Environment Surrounding the First Stars
Hajime Susa
Online at http://www.int.washington.edu/talks/WorkShops/int_06_2a 2006年7月
単著
担当区分：筆頭著者
-
Environment Surrounding the First Stars
須佐 元
The First Stars and Evolution of the Early Universe 2006年7月
-
Secondary star formation in a population III object
Susa H, Umemura M.
Astrophysical Journal 645 ( 2 II ) 2006年7月
共著
We explore the possibility of subsequent star formation after a first star forms in a Population III object, by focusing on the radiation-hydrodynamic (RHD) feedback caused by ionizing photons, as well as H2-dissociating photons. For this purpose, we perform three-dimensional RHD simulations in which the radiative transfer of ionizing photons and H2-dissociating photons from a first star is self-consistently coupled with hydrodynamics based on a smoothed particle hydrodynamics method. It is shown that density peaks above a threshold density can keep collapsing, owing to the shielding of H2-dissociating radiation by an H2 shell formed ahead of a D-type ionization front. But, below the threshold density an M-type ionization front with a shock propagates, and density peaks are radiation-hydrodynamically evaporated by the shock. The threshold density depends on the distance from the source star and is ~102 cm-3 for a source distance of 30 pc. Taking into consideration that the extent of a Population III object is ~100 pc and the density peaks within it have densities of 102-104 cm-3, it is concluded that secondary star formation is possible in the broad regions of a Population III object....
DOI： 10.1086/506275
-
Smoothed particle hydrodynamics coupled with radiation transfer
Susa H.
Publications of the Astronomical Society of Japan 58 ( 2 ) 445 - 460 2006年4月
単著
We have constructed a brand-new radiation hydrodynamics solver based upon Smoothed Particle Hydrodynamics, which works on a parallel computer system. The code is designed to investigate the formation and evolution of first-generation objects at z ≳ 10, where the radiative feedback from various sources plays important roles. The code can compute the fraction of chemical species e, H+, H, H-, H2, and H+2 by by fully implicit time integration. It also can deal with multiple sources of ionizing radiation, as well as radiation at Lyman-Werner band. We compare the results for a few test calculations with the results of one-dimensional simulations, in which we find good agreements with each other. We also evaluate the speedup by parallelization, which is found to be almost ideal, as long as the number of sources is comparable to the number of processors....
-
Formation of dwarf galaxies in reionized universe with heterogeneous multicomputer system
Boku T, Susa H, Onuma K, Umemura M, Sato M, Takahashi D.
International Journal for Multiscale Computational Engineering 4 ( 2 ) 281 - 289 2006年
-
The effects of early reionization on the formation of dwarf galaxies
Hajime Susa, Masayuki Umemura
IAU Colloquium Proceedings of the international Astronomical Union 198 147 - 150 2005年11月
-
Dissipation of Magnetic Fields in Primordial Gas Clouds
Hideki Maki, Hajime Susa
AIP Conference Proceedings 784 808 - 818 2005年9月
共著
DOI： 10.1063/1.2077254
-
The structure and evlution of early cosmological HII regions
須佐元
Astrophysical Journal Volume 613 ( 631 ) 2004年11月
単著
-
Formation of Dwarf Galaxies during the Cosmic Reionization
須佐元
Astrophysical Journal Volume 600 ( 1 ) 2004年11月
単著
-
Dissipation of Magnetic Flux in Primordial Clouds
須佐元
Astrophysical Journal Volume 609 ( 467 ) 2004年11月
単著
-
The Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halos
須佐元
Astrophysical Journal Volume 610 ( L5 ) 2004年11月
単著
-
The structure and evolution of early cosmological H II regions
Kitayama T, Yoshida N, Susa H, Umemura M.
Astrophysical Journal 613 ( 2 I ) 631 - 645 2004年10月
共著
We study the formation and evolution of H II regions around the first stars formed at redshifts z=10-30. We use a one-dimensional Lagrangian hydrodynamics code that self-consistently incorporates radiative transfer and nonequilibrium primordial gas chemistry. The star-forming region is defined as a spherical dense molecular gas cloud with a Population III star embedded at the center. We explore a large parameter space by considering, as plausible early star-forming sites, dark matter halos of mass Mhalo=105-108 Msolar, gas density profiles with a power-law index w=1.5-2.25, and metal-free stars of mass Mstar=25-500 Msolar. The formation of the H II region is characterized by initial slow expansion of a weak D-type ionization front near the center, followed by rapid propagation of an R-type front throughout the outer gas envelope. We find that the transition between the two front types is indeed a critical condition for the complete ionization of halos of cosmological interest. In small-mass (<~106 Msolar) halos, the transition takes place within a few 105 yr, yielding high escape fractions (>80%) of both ionizing and photodissociating photons. The gas is effectively evacuated by a supersonic shock, with the mean density within the halo decreasing to <~1 cm-3 in a few million years. In larger mass (>~107 Msolar) halos, the ionization front remains to be of D-type over the lifetime of the massive star, the H II region is confined well inside the virial radius, and the escape fractions are essentially zero. We derive an analytic formula that reproduces well the results of our simulations for the critical halo mass below which the gas is completely ionized. We discuss immediate implications of the present results for the star formation history and early reionization of the universe....
DOI： 10.1086/423313
-
Dissipation of magnetic flux in primordial gas clouds
Maki H, Susa H.
Astrophysical Journal 609 ( 2 I ) 467 - 473 2004年7月
共著
We report on the strength of the seed magnetic flux of the accretion disk surrounding Population III stars. Themagnetic field in accretion disks might play an important role in the transport of angular momentum because of the turbulence induced by magnetorotational instability (MRI). On the other hand, since the primordial star-forming clouds contain no heavy elements or grains, they experience a very different thermal history and magnetic-field-dissipation history in the course of their gravitational contraction from those in the present-day star-forming molecular clouds. In order to assess the magnetic field strength in the accretion disk of Population III stars, we calculate the thermal history of the primordial collapsing clouds and investigate the coupling of the magnetic field with the primordial gas. As a result, we find that the magnetic field strongly couples with the primordial gas cloud throughout the collapse, i.e., the magnetic field is frozen to the gas, as long as the initial field strength satisfies B<~10-5(NH/103cm-3)0.55G....
DOI： 10.1086/421103
-
Quasar Mesolensing as a Probe of CDM Substructures
須佐 元
Dark Matter in Galaxies 2004年7月
単著
We investigate methods to detect numerous CDM substructures around galaxies predicted by cosmological N-body simulations via medium-scale gravitational-lens effect. Magnification anomaly due to such substructures have already been noted by some researchers and applied to gravitationally lensed quasars. In this presentation we focus on other phenomena produced by gravitational lens effects multiple images and the time delays between images at the same situation i.e. the lens is a galaxy and the source is a distant quasar. Here we have included the effect of the galaxy itself so-called ""Chang-Refsdal lens"" situation and estimated image separations and the time delays. The expected values are from 1 to 100 mas for the image separation and from 10 to 1000 sec for the time delay with slight lens model dependence. Event rate for such phenomena is the order of percent but such signals in one image of a gravitationally lensed quasar must be a direct evidence for the numerous CDM substructures. Image separations and time delays are reflect mass of the lens and our methods will provide better constraints on the substructres compared with magnification anomaly....