Hajime Susa

The Structure and Evolution of Early Cosmological H II Regions

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 M☉, gas density profiles with a power-law index w = 1.5-2.25, and metal-free stars of mass Mstar = 25-500 M☉. 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 M☉) 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 M☉) 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.

Cosmological radiative transfer codes comparison project - I. The static density field tests

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 H II region expansion; (2) H II 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.

Formation of Dwarf Galaxies during the Cosmic Reionization

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.

Cosmological radiative transfer comparison project - II. The radiation-hydrodynamic tests

The development of radiation hydrodynamical methods that are able to follow gas dynamics and radiative transfer (RT) self-consistently is key to the solution of many problems in numerical astrophysics. Such fluid flows are highly complex, rarely allowing even for approximate analytical solutions against which numerical codes can be tested. An alternative validation procedure is to compare different methods against each other on common problems, in order to assess the robustness of the results and establish a range of validity for the methods. Previously, we presented such a comparison for a set of pure RT tests (i.e. for fixed, non-evolving density fields). This is the second paper of the Cosmological Radiative Transfer Comparison Project, in which we compare nine independent RT codes directly coupled to gas dynamics on three relatively simple astrophysical hydrodynamics problems: (i) the expansion of an H ii region in a uniform medium, (ii) an ionization front in a 1/r2 density profile with a flat core and (iii) the photoevaporation of a uniform dense clump. Results show a broad agreement between the different methods and no big failures, indicating that the participating codes have reached a certain level of maturity and reliability. However, many details still do differ, and virtually every code has showed some shortcomings and has disagreed, in one respect or another, with the majority of the results. This underscores the fact that no method is universal and all require careful testing of the particular features which are most relevant to the specific problem at hand.

Fragmentation of the Primordial Gas Clouds and the Lower Limit on the Mass of the First Stars

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 that is formed from the primordial gas cloud and essentially determined by the Chandrasekhar mass.

THE MASS OF THE FIRST STARS

We perform a three-dimensional radiation hydrodynamics simulation to investigate the formation of the first stars from the initial collapse of a primordial gas cloud to the formation and growth of protostars. The simulation is integrated until ~0.1?Myr after the formation of the primary protostar, by which time the protostars have already settled onto the main sequence. This work represents the first attempt at simulating the first episodes of star formation, taking into account the ultraviolet radiative feedback effect from multiple protostars as well as the three-dimensional effects of the fragmentation of the accretion disk. We find that the mass accretion onto Population?III protostars is significantly suppressed by their radiative feedback. As a result, we find five stars formed in this particular simulation. The final masses of the stars are 60 M ?, including a star of 4.4 M ?. Formation of such a star hints at the existence of even lower-mass stars that would live today.

Secondary Star Formation in a Population III Object

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.

The Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halos

Recent observations of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe strongly suggest that the reionization of the universe took place quite early (z ~ 17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from a substructure problem in which more subgalactic halos are produced in the Local Group than dwarf galaxies. In this Letter, as a potential mechanism for solving this problem, we consider the feedback effects of early reionization on the formation of small-scale structures. For this purpose, we perform three-dimensional radiation hydrodynamic simulations, incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is devastating for low-mass systems with Mvir 108 M? or vcirc 20 km s-1, and almost all gas is photoevaporated in more than 95% of low-mass systems. Such a strong negative feedback on the formation of low-mass galaxies may solve the substructure problem and support the picture that Local Group dwarf galaxies are descendants of the more massive halos that experienced and survived tidal stripping.

DISSIPATION OF MAGNETIC FLUX IN PRIMORDIAL GAS CLOUDS

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/103 cm-3)0.55 G.

Photodissociation feedback of population III stars onto neighboring prestellar cores

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.