Tamon Suwa

Stellar Abundances for the Galactic Archeology (SAGA) Database — Compilation of the Characteristics of Known Extremely Metal-Poor Stars

We describe the construction of a database of extremely metal-poor (EMP) stars in the Galaxy. Our database contains detailed elemental abundances, reported equivalent widths, atmospheric parameters, photometry, and binarity status, compiled from papers in the literature that report on studies of EMP halo stars with [Fe=H] �� 2.5. The compilation procedures for this database were designed to assemble data effectively from electronic tables available from online journals. We have also developed ad ata retrieval system that enables data searches by various criteria and illustrations to explore relationships between stored variables. Currently, our sample includes 1212 unique stars (many of which are studied by more than one group) with more than 15000 individual reported elemental abundances, covering relevant papers published by 2007 December. We discuss the global characteristics of the present database, as revealed by the EMP stars observed to date. For stars with [Fe=H] �� 2.5, the number of giants with reported abundances is larger than that of dwarfs by a factor of two. The fraction of carbon-rich stars (among the sample for which the carbon abundance is reported) amounts to � 30% for [Fe=H] �� 2.5. We find that known binaries exhibit different distributions of the orbital period, according to whether they are giants or dwarfs, and also as a function of the metallicity, although the total sample of such stars is still quite small.

Cluster Morphology as a Test of Different Cosmological Models

We investigate how cluster morphology is affected by the cosmological constant in low-density universes. Using high-resolution cosmological N-body/smoothed particle hydrodynamics simulations of flat cold dark matter (ΛCDM; Ω0 = 0.3, λ0 = 0.7) and open cold dark matter (OCDM; Ω0 = 0.3, λ0 = 0) universes, we calculate statistical indicators to quantify the irregularity of the cluster morphologies. We study axial ratios, center shifts, cluster clumpiness, and multipole-moment power ratios as indicators for the simulated clusters at z = 0 and 0.5. Some of these indicators are calculated for both the X-ray surface brightness and projected mass distributions. In ΛCDM all these indicators tend to be larger than those in OCDM at z = 0. This result is consistent with the analytical prediction of Richstone, Loeb, & Turner, that is, clusters in ΛCDM are formed later than in OCDM and have more substructure at z = 0. We perform a Kolmogorov-Smirnov test on each indicator for these two models. We then find that the results for the multipole-moment power ratios and the center shifts for the X-ray surface brightness are under the significance level (5%). Our results also show that these two cosmological models can be distinguished more clearly at z = 0 than at z = 0.5 by these indicators.

Protoclusters in the ΛCDM Universe

We compare the highly clustered populations of very high redshift galaxies with protoclusters identified numerically in a standard ΛCDM universe (Ω0 = 0.3, λ0 = 0.7) simulation. We evolve 2563 dark matter particles in a comoving box of side 150 h-1 Mpc. By the present day there are 63 cluster-sized objects of mass in excess of 1014 h-1 M☉ in this box. We trace these clusters back to higher redshift, finding that their progenitors at z = 4-5 are extended regions of typically 20-40 Mpc (comoving) in size, with dark halos of mass in excess of 1012 h-1 M☉ and are overdense by typically 1.3-13 times the cosmological mean density. Comparison with observations of Lyα-emitting galaxies at z = 4.86 and 4.1 indicates that the observed excess clustering is consistent with that expected for a protocluster region if Lyα emitters typically correspond to massive dark halos of more than 1012 h-1 M☉. We give a brief discussion of the relation between the high-redshift concentration of massive dark halos and present-day rich clusters of galaxies.

Formation and radiative feedback of first objects and first galaxies

first luminous objects with the aid of dark matter cusps. Therefore, the mass of first objects is smaller by roughly two orders of magnitude than in the previous prediction. This implies that the number of Pop III stars formed in the early universe could be significantly larger than hitherto thought. Secondly, the feedback by photo-ionization and photo-dissociation photons in the first objects is explored with radiation hydrodynamic simulations, and it is demonstrated that multiple stars can form in a 10 5 M⊙ halo. Thirdly, the fragmentation of an accretion disk around a primordial protostar is explored with photo-dissociation feedback. As a result, it is found that the photo-dissociation can reduce the mass accretion rate onto protostars. Also, protostars as small as 0.8M⊙ may be ejected and evolve with keeping their mass, which might be detected as “real first stars” in the Galactic halo. Finally, state-of-theart radiation hydrodynamic simulations are performed to investigate the internal ionization of first galaxies and the escape of ionizing photons. We find that UV feedback by forming massive stars enhances the escape fraction even in a halo as massive as > 6 × 10 9 M⊙, while it reduces the star formation rate significantly. This may have a momentous impact on the cosmic reionization.

Impacts of diffuse UV radiation on the secondary Population III star formation

Using thee‐dimensional radiation hydrodynamics (RHD) simulations, we explore the radiative feedback process on the secondary Population III (Pop III) star formation. As recently shown by Umemura et al. (2010), a density peak can independently collapse in the distance of ∼60 pc from a precollapsed density peak. Since Pop III stars are expected to be very massive, strong UV radiation from a Pop III star born in a precollapsed density peak is likely to play a crucial role on the fate of an adjacent density peak. By RHD simulations, we find that the adjacent peak can survive although a shock associated with ionization front strips a large amount of gas. The surviving cloud is never ionized, but H2 molecule fraction is higher than that in usual Pop III cloud. Based on the obtained physical quantities of the cloud, we also discuss the mass of the secondary Pop III star.

The Collapse of First Objects driven by Dark Matter Cusps

A hybrid simulator FIRST is built up to perform three‐dimensional high‐resolution simulations on the first objects in the universe. As a result of simulations with FIRST, we find that dark matter cusps can reduce the mass of first collapsed objects to a level of 103 Msun in baryon. It is by more than one order of magnitude smaller than previous simulations. Hence, much more Pop III stars are expected than the previous prediction.

The Impact of Ultraviolet Radiation on Secondary Pop III Star Formation

Using thee‐dimensional radiation hydrodynamics (RHD) simulations, we explore the radiative feedback effects by a Population III (Pop III) star on second star formation. We find that the adjacent peak can survive, although a shock associated with ionization front strips a large amount of gas. The surviving cloud is never ionized, but H2 fractions become large due to radiation hydrodynamic effects. These effects lower mass accretion rates onto the adjacent peak, and consequently the second star is likely to be less massive.

FIRST Project: Formation and Feedback of the First Stars

We introduce the “FIRST Project”, where high‐resolution 3D radiation hydrodynamic simulations of first star formation are performed with a newly developed dedicated computer having a peak speed of 36.1 Tflops. By solving the hydrodynamics consistently with the radiative transfer of UV radiation from first stars, we explore the radiation hydrodynamic feedback in first generation objects, and consider the star formation efficiency of the first stars.

Radiative Transfer SPH Simulations of UV Feedback on Pop III Star Formation

To explore the radiative feedback from first stars on the subsequent Pop III star formation in a Lambda‐CDM universe, we perform high‐resolution cosmological SPH simulations coupled with radiative transfer of UV radiation, using FIRST simulator. In the simulations, the baryon mass resolution is 0.3 M⊙ and the number of particles is several 105 respectively for baryon and dark matter component. We assume that several collapsing cores form in a Pop III object and one of them evolves into a first star, and then investigate the evolution of other collapsing cores with paying special attention to the radiation hydrodynamic feedback. As a result, it is shown that other collapsing cores are impervious to the negative radiative feedbacks of H2 photodissociation and photoevapolation, but can continue to collapse due to the shielding of UV radiation, consistent with the results of Susa & Umemura (2006). This result implies that all collapsing cores in a Pop III object can evolve into stars.

High Resolution P3M‐GRAPE‐SPH Simulations of PopIII Star Formation

In order to investigate the mass distribution of Population III stars, we have developed high‐resolution cosmological hydrodynamic simulations in a standard ΛCDM universe. We use a Particle‐Particle‐Particle‐Mesh (P3M) scheme for gravity force calculations, where the PP part is calculated by GRAPE (special purpose board for gravity calculation). The baryon component is treated by Smoothed Particle Hydrodynamics (SPH) method.The simulations are performed on a newly developed dedicated PC‐cluster system named FIRST simulator with the peak speed of 36 T flops at the University of Tsukuba. Using this simulator, we perform high‐resolution simulations with 2×107 of dark matter and SPH particles, respectively. With this large number of particles, sub‐solar mass resolution for baryon particles can be achieved all over the simulation box. Therefore, we can resolve small scale density fluctuations, which may play an important role in the formation of intermediate‐mass Pop III stars formation.