Shingo Hirano

One Hundred First Stars : Protostellar Evolution and the Final Masses

We perform a large set of radiation hydrodynamic simulations of primordial star formation in a fully cosmological context. Our statistical sample of 100 First Stars shows that the first generation of stars has a wide mass distribution M popIII = 10 ~ 1000 M ☉. We first run cosmological simulations to generate a set of primordial star-forming gas clouds. We then follow protostar formation in each gas cloud and the subsequent protostellar evolution until the gas mass accretion onto the protostar is halted by stellar radiative feedback. The accretion rates differ significantly among the primordial gas clouds that largely determine the final stellar masses. For low accretion rates, the growth of a protostar is self-regulated by radiative feedback effects, and the final mass is limited to several tens of solar masses. At high accretion rates the protostars outer envelope continues to expand, and the effective surface temperature remains low; such protostars do not exert strong radiative feedback and can grow in excess of 100 solar masses. The obtained wide mass range suggests that the first stars play a variety of roles in the early universe, by triggering both core-collapse supernovae and pair-instability supernovae as well as by leaving stellar mass black holes. We find certain correlations between the final stellar mass and the physical properties of the star-forming cloud. These correlations can be used to estimate the mass of the first star from the properties of the parent cloud or of the host halo without following the detailed protostellar evolution.

Primordial star formation under the influence of far ultraviolet radiation: 1540 cosmological haloes and the stellar mass distribution

We perform a large set of cosmological simulations of early structure formation and follow the formation and evolution of 1540 star-forming gas clouds to derive the mass distribution of primordial stars. The star formation in our cosmological simulations is characterized by two distinct populations, the so-called Population III.1 stars and primordial stars formed under the influence of far ultraviolet (FUV) radiation (Population III.2D stars). In this work, we determine the stellar masses by using the dependences on the physical properties of star-forming cloud and/or the external photodissociating intensity from nearby primordial stars, which are derived from the results of two-dimensional radiation hydrodynamic simulations of protostellar feedback. The characteristic mass of the Pop III stars is found to be a few hundred solar masses at z ~ 25, and it gradually shifts to lower masses with decreasing redshift. At high redshifts z > 20, about half of the star-forming gas clouds are exposed to intense FUV radiation and thus give birth to massive Pop III.2D stars. However, the local FUV radiation by nearby Pop III stars becomes weaker at lower redshifts, when typical Pop III stars have smaller masses and the mean physical separation between the stars becomes large owing to cosmic expansion. Therefore, at z < 20, a large fraction of the primordial gas clouds host Pop III.1 stars. At z =< 15, the Pop III.1 stars are formed in relatively cool gas clouds due to efficient radiative cooling by H_2 and HD molecules; such stars have masses of a few x 10 Msun. Since the stellar evolution and the final fate are determined by the stellar mass, Pop III stars formed at different epochs play different roles in the early universe.

Formation of Massive Primordial Stars: Intermittent UV Feedback with Episodic Mass Accretion

We present coupled stellar evolution (SE) and 3D radiation-hydrodynamic (RHD) simulations of the evolution of primordial protostars, their immediate environment, and the dynamic accretion history under the influence of stellar ionizing and dissociating UV feedback. Our coupled SE-RHD calculations result in a wide diversity of final stellar masses covering 10 Msun

Room-temperature nanowire ultraviolet lasers: An aqueous pathway for zinc oxide nanowires with low defect density

\lesssim M_* \lesssim

COSMOLOGICAL SIMULATIONS OF EARLY BLACK HOLE FORMATION: HALO MERGERS, TIDAL DISRUPTION, AND THE CONDITIONS FOR DIRECT COLLAPSE

1000 Msun. The formation of very massive (

WEAKLY INTERACTING MASSIVE PARTICLE DARK MATTER AND FIRST STARS: SUPPRESSION OF FRAGMENTATION IN PRIMORDIAL STAR FORMATION

\gtrsim

Radiative Cooling Implementations in Simulations of Primordial Star Formation

250 Msun) stars is possible under weak UV feedback, whereas ordinary massive (a few x 10 Msun) stars form when UV feedback can efficiently halt the accretion. This may explain the peculiar abundance pattern of a Galactic metal-poor star recently reported by Aoki et al. (2014), possibly the observational signature of very massive precursor primordial stars. Weak UV feedback occurs in cases of variable accretion, in particular when repeated short accretion bursts temporarily exceed 0.01 Msun/yr, causing the protostar to inflate. In the bloated state, the protostar has low surface temperature and UV feedback is suppressed until the star eventually contracts, on a thermal adjustment timescale, to create an HII region. If the delay time between successive accretion bursts is sufficiently short, the protostar remains bloated for extended periods, initiating at most only short periods of UV feedback. Disk fragmentation does not necessarily reduce the final stellar mass. Quite the contrary, we find that disk fragmentation enhances episodic accretion as many fragments migrate inward and are accreted onto the star, thus allowing continued stellar mass growth under conditions of intermittent UV feedback. This trend becomes more prominent as we improve the resolution of our simulations. We argue that simulations with significantly higher resolution than reported previously are needed to derive accurate gas mass accretion rates onto primordial protostars.

Electrical and Thermal Transport Properties in Layer-Structured (ZnO)mIn2O3 (m=5 and 9) Ceramics

We report the realization of a low-temperature aqueous pathway for the chemical synthesis of zinc oxide (ZnO) nanowires with low defect density and their room-temperature ultraviolet lasing behavior at low pump fluence. The concentration of solutes determined not only the size of individual nanowires, which influences their optical waveguiding behavior, but also their lattice defect density, which affects the efficiency of ultraviolet emission. The optimal synthesis conditions led to low-temperature growth of ZnO nanowires that showed room-temperature ultraviolet lasing at a low threshold of pump fluence. Based on our experimental results and optical waveguide theory, we report two important factors for realizing high-quality ZnO nanowires that show room-temperature ultraviolet lasing via a low-temperature aqueous approach: control of the density of defects generated in aqueous solutions and the optimal microstructure of the grown nanowires to produce strong optical confinement.

Two-Dimensional Ceramic Photonic Crystals Fabricated by a Solution Method

Gravitational collapse of a massive primordial gas cloud is thought to be a promising path for the formation of supermassive blackholes in the early universe. We study conditions for the so-called direct collapse (DC) blackhole formation in a fully cosmological context. We combine a semi-analytic model of early galaxy formation with halo merger trees constructed from dark matter

Supersonic gas streams enhance the formation of massive black holes in the early universe

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