Papers - SUSA Hajime
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Collapse of low-mass clouds in the presence of a UV radiation field
Susa H, Kitayama T.
Monthly Notices of the Royal Astronomical Society 317 ( 1 ) 175 - 178 2000.9
Joint Work
The collapse of marginally Jeans-unstable primordial gas clouds in the presence of a UV radiation field is discussed. Assuming that the dynamical collapse proceeds approximately in an isothermal self-similar fashion, we investigate the thermal evolution of the collapsing central core until H2 cooling dominates photoheating and the temperature drops to below 104K. Consequently, the mass of the cooled core is evaluated as Mcool=3.6×106Msolar (I21/1)-0.32. This scale depends only on the incident UV intensity, and provides a lower limit to the mass of collapsed objects in the UV radiation field....
DOI： 10.1046/j.1365-8711.2000.03616.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Ultraviolet-background-induced bifurcation of galactic morphology
Susa H, Umemura M.
Monthly Notices of the Royal Astronomical Society 316 ( 2 ) 2000.8
Joint Work
Based upon a novel paradigm of galaxy formation under the influence of an ultraviolet background, the evolutionary bifurcation of pre-galactic clouds is compared with observations of elliptical and spiral galaxies. The theory predicts that the dichotomy between dissipational and dissipationless galaxy formation stems from the degree of self-shielding from the ultraviolet background radiation. This is demonstrated on a bifurcation diagram of collapse epochs versus masses of pre-galactic clouds. Using the observed properties, the collapse epochs and mass are assessed for each type of galaxy. By direct comparison of the theory with observations, it turns out that the theoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the ultraviolet background radiation could play a profound role in the differentiation of galactic morphology into the Hubble sequence....
DOI： 10.1046/j.1365-8711.2000.03732.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of Primordial Galaxies under Ultraviolet Background Radiation
The Astrophysical Journal 2000.7
Single Work
The pancake collapse of pregalactic clouds under UV background radiation is explored with a one-dimensional sheet model. Here, attention is concentrated on elucidating the basic physics on the thermal evolution of pregalactic clouds exposed to diffuse UV radiation. So, we treat accurately the radiation transfer for the ionizing photons, with solving chemical reactions regarding hydrogen molecules as well as atoms. The self-shielding against UV radiation by H2 Lyman-Werner bands, which regulates the photodissociation of hydrogen molecules, is also taken into account. As a result, it is found that when the UV background radiation is at a level of 10-22(ν/νL)-1 ergs s-1 cm-2 Hz-1 sr-1, the cloud evolution bifurcates with a critical mass as MSB=2.2×1011 Msolar[(1+zc)/5]-4.2, where zc is the final collapse epoch. A cloud more massive than MSB cools below 5×103 K owing to H2 line emission at the pancake collapse and would undergo the initial starburst. The pancake possibly evolves into a virialized system in a dissipationless fashion. Consequently, this leads to the dissipationless galaxy formation at 3<~zc<~10. A cloud less massive than MSB cannot cool by H2 emission shortly after the pancake collapse but could cool in the course of shrinking to the rotation barrier. This is likely to lead to the dissipational galaxy formation at relatively low redshifts as 0<~zc<~4. The present results provide a solid physical mechanism that controls the star formation efficiency in the pregalactic clouds. In the context of a standard CDM cosmology, MSB lies between 1 σ and 2 σ density fluctuations....
DOI： 10.1086/309039
Other Link： http://orcid.org/0000-0002-3380-5302
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Radiation-hydrodynamical collapse of pre-galactic clouds in the ultraviolet background
Kitayama T, Tajiri Y, Umemura M, Susa H, Ikeuchi S.
Monthly Notices of the Royal Astronomical Society 315 ( 1 ) 2000.6
Joint Work
To explain the effects of the ultraviolet (UV) background radiation on the collapse of pre-galactic clouds, we implement a radiation-hydrodynamical calculation, combining one-dimensional spherical hydrodynamics with an accurate treatment of the radiative transfer of ionizing photons. Both absorption and scattering of UV photons are explicitly taken into account. It turns out that a gas cloud contracting within the dark matter potential does not settle into hydrostatic equilibrium, but undergoes run-away collapse even under the presence of the external UV field. The cloud centre is shown to become self-shielded against ionizing photons by radiative transfer effects before shrinking to the rotation barrier. Based on our simulation results, we further discuss the possibility of H2 cooling and subsequent star formation in a run-away collapsing core. The present results are closely relevant to the survival of subgalactic Population III objects as well as to metal injection into intergalactic space....
DOI： 10.1046/j.1365-8711.2000.03589.x
Other Link： http://orcid.org/0000-0002-3380-5302
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Photoionization and dynamical evolution of pregalactic clouds
Nuclear Physics B Proceedings Supplements 2000.1
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Formation of primordial galaxies under ultraviolet background radiation
Susa H, Umemura M.
Astrophysical Journal 537 ( 2 PART 1 ) 578 - 588 2000
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On the Formation of Primordial Stars and Pop III Luminous Objects
The First Stars 2000
Single Work
We investigate the formation site of the primordial stars, the formation process of them and the feedback on the host clouds. First, the `cooling diagram' for cosmologicallow mass objects is shown. We assess the cooling rate taking into account the contribution of H2, which is not in chemical equilibrium generally. Next, we show the results of the hydrodynamical simulation for primordial star formation, which includes radiative transfer and the chemistry of H2. Moreover, the feed back by the first generation massive stars is considered. The main feedback consists of two different processes, UV radiation from the stars and energy input by SNe. Finally, we investigate the formation condition of luminous objects and the mass of the Pop III luminous objects is estimated to be several times 107 M_☉....
DOI： 10.1007/10719504_49
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation of Primordial Stars and Evolution of Primordial Gas Cloud
Star Formation 1999 1999.12
Single Work
We investigate the evolution of cosmological low mass (low virial temperature) clouds and the formation of the first stars. We also estimate the feedback by the first generation stars to the host clouds. First, the `cooling diagram' for low mass objects is shown. We assess the cooling rate taking into account the contribution of Hydrogen molecules, which is not in chemical equilibrium generally, with a simple argument of time scales. The reaction rates and the cooling rate are taken from the recent results by Galli and Palla (1998). Next, we show the simulation of the dynamical collapse of metal free protostellar clouds and formation of protostars. We performed hydrodynamical calculations for spherically symmetric clouds taking account of radiative transfer of the molecular hydrogen lines and the continuum, as well as of chemistry of the molecular hydrogen. The collapse is non-homologous and proceed almost self-similarly like Larson-Penston similarity solution. The effective `adiabatic index' at the center is about 1.1 almost throughout the collapse. The mass accretion rate is estimated to be very large and the massive star formation is expected. Finally, we investigate the feed back by the first generation massive stars. The main feedback consists of two different processes, UV radiation from the stars and energy input by SNe. The feedback from the formed stars on their own host cloud is very serious....
Other Link： http://orcid.org/0000-0002-3380-5302
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Origin of Hubble Sequence --- Bifurcation into Spiral and Elliptical Galaxies ---
Star Formation 1999 1999.12
Single Work
Evolutionary bifurcation of cosmological density perturbation is discussed according to the galaxy formation theory proposed by Susa & Umemura (1999). We plot the observed elliptical and spiral galaxies on the cosmological initial condition plane (collapse epoch v.s. mass), with the help of virial theorem. On the initial condition plane, the boundary between the elliptical and spiral galaxies coincide very well with the predicted line by the theory....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation and Disruption of Cosmological Low-Mass Objects
The Astrophysical Journal 1999.10
Single Work
We investigate the evolution of cosmological low-mass (low virial temperature) objects and the formation of the first luminous objects. First, the ``cooling diagram'' for low-mass objects is shown. We assess the cooling rate taking into account the contribution of H2, which is not in chemical equilibrium generally, with a simple argument of timescales. The reaction rates and the cooling rate of H2 are taken from the recent results by Galli & Palla. Using this cooling diagram, we also estimate the formation condition of luminous objects taking into account the supernova (SN) disruption of virialized clouds. We find that the mass of the first luminous object is several times 107 Msolar because smaller objects may be disrupted by the SNe before they become luminous. Metal pollution of low-mass (Lyα) clouds is also discussed. The resultant metallicity of the clouds is Z/Zsolar~10-3....
DOI： 10.1086/312277
Other Link： http://orcid.org/0000-0002-3380-5302
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3D Radiative Transfer Calculations on the Cosmic ReionizationCD
Numerical Astrophysics 1999
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Fragmentation of the Primordial Gas Clouds
Cosmological Parameters and the Evolution of the Universe 1999
Single Work
We discuss the fragmentation of primordial gas clouds in the universe after decoupling. Comparing the timescale of collapse with that of fragmentation, we obtain the typical mass of a fragment both numerically and analytically. We show that the estimated mass gives the minimum mass of a fragment which is formed from the primordial gas cloud and is essentially determined by the Chandrasekhar mass....
Other Link： http://orcid.org/0000-0002-3380-5302
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Formation and disruption of cosmological low-mass objects
Nishi R, Susa H.
Astrophysical Journal 523 ( 2 PART 2 ) 1999
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Evolution of Primordial Protostellar Clouds
Cosmological Parameters and the Evolution of the Universe 1999
Single Work
We present results of the hydrodynamical calculation of collapsing primordial proto-stellar clouds. In these calculations, the radiative transfer of molecular hydrogen lines is considered. The molecular hydrogen line cooling is so effective, even when a cold and optically thick envelope is present, that the cloud can collapse at almost free-fall rate. This phenomenon is peculiar to line cooling. A line is not infinitely sharp, but has some width mainly due to thermal Doppler shift. This means that even if the cloud becomes optically thick at the line centre,it is still optically thin at the line edge. Therefore photons can still escape from the clouds....
Other Link： http://orcid.org/0000-0002-3380-5302
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Photoionization and Dynamical Evolution of Mini-PancakesCD
Numerical Astrophysics 1999
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Photoionization and Dynamical Evolution of Pregalactic Clouds
19th Texas Symposium on Relativistic Astrophysics and Cosmology 1998.12
Single Work
We investigate thermal and hydrodynamical evolution of collapsing pancake in the era of galaxy formation. We solve detailed chemical rate equations for six species, e, H, H^+, H^-, H^+_2 and H_2. We compute photoionization and photodissociation rates by the UV radiation field. We integrate the radiation transfer equation for the ionizing photons (h nu >= 13.6eV) strictly. When the pancake is irradiated by the UV background radiation field (J21 = 0.1, J_nu propto nu^{-alpha}, alpha = 1), we find a critical thickness with a given initial averaged density bar{n}_ini. It is lambda_cool = 1.1 Mpc (bar{n}_ini/1.0 times 10^{-4} cm^{-3})^{-0.8} that determines whether the sheet will cool or not. If the initial thickness of the sheet is larger than lambda_cool, the sheet collapses and cool below 5 times 10^3K by H_2 rovibrational line emission. Otherwise, the temperature of the sheet is high (T > 10^4 K) throughout the evolution, even if it collapse....
Other Link： http://orcid.org/0000-0002-3380-5302
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Thermal and dynamical evolution of primordial gas clouds: On the formation of first luminous objects
Nishi R, Susa H, Uehara H, Yamada M, Omukai K.
Progress of Theoretical Physics 100 ( 5 ) 881 - 903 1998.11
Joint Work
We investigate the thermal and dynamical evolution of primordial gas clouds in the universe after decoupling. Comparing the time scale of dynamical evolution with that of fragmentation, we can estimate the typical fragmentation scale. We propose the following scenario of the formation process of first luminous objects consisting of large number stars. First, by pancake collapse of the overdensity regions in the expanding universe or collision between clouds in potential wells, quasi-plane shocks form. If the shock-heated temperature is higher than about 104 K, the post-shock gas cools down to several hundred K by H2 line cooling, and the shock-compressed layer fragments into filamentary clouds. The filamentary cloud collapses dynamically once more and fragments into cloud cores. Finally, a primordial star forms in a cloud core. We show that the minimum mass of the first star is essentially determined by the Chandrasekhar mass. Also, we investigate the dynamical collapse of cloud cores by numerical simulation and show that the evolution paths of the central regions of the cores depend only very weakly on the total core mass. After mass accretion, a massive star may be formed in a core, since the estimated mass accretion rate is very large. In such a case, it may be possible for many massive stars form almost simultaneously. Then the clouds can be luminous objects. On the other hand, if the shock-heated temperature is lower, effective star formation is delayed significantly....
DOI： 10.1143/PTP.100.881
Other Link： http://orcid.org/0000-0002-3380-5302
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Evolution of primordial protostellar clouds quasi-static analysis
Omukai K, Nishi R, Uehara H, Susa H.
Progress of Theoretical Physics 99 ( 5 ) 747 - 761 1998.5
Joint Work
The contraction processes of metal-free molecular clouds of starlike mass (or cloud cores) are investigated. We calculate radiative transfer of the H2 lines and examine quasi-static contraction with radiative cooling. Comparing two time-scales, the free-fall time tff and the time-scale of quasi-static contraction tqsc (~ tcool, the cooling time) of these cores, we find that the ratio of the two time-scales tff/tqsc, i.e., the efficiency of cooling, becomes larger with contraction even under the existence of cold and opaque envelopes. In particular, for fragments of primordial filamentary clouds, for which tff ~ tqsc at the fragmentation epoch, they collapse dynamically in the free-fall time-scale. This efficiency of cooling is unique to line cooling....
DOI： 10.1143/PTP.99.747
Other Link： http://orcid.org/0000-0002-3380-5302
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On the mass of first generation stars and formation of MACHOS
Memorie della Societa Astronomica Italiana 1998
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Thermal evolution in post-shock layer
Memorie della Societa Astronomica Italiana 1998