Abstract
We use cosmological hydrodynamic simulations with unprecedented resolution to study the formation of primordial stars in an ionized gas at high redshifts. Our approach includes all the relevant atomic and molecular physics to follow the thermal evolution of a prestellar gas cloud to very high densities of ~1018 cm-3. We locate a star-forming gas cloud within a reionized region in our cosmological simulation. The gas cloud cools down to a few tens of kelvins by HD line cooling, and this is lower than possible by H2 cooling only. Owing to the low temperature, the first runaway collapse is triggered when the gas cloud's mass is ~40 M☉. We show that the cloud core remains stable against chemothermal instability and also against gravitational deformation throughout its evolution. Consequently, a single protostellar seed is formed, which accretes the surrounding hot gas at the rate 10-3 M☉ yr-1. We carry out protostellar evolution calculations using the inferred accretion rate. The resulting mass of the star when it reaches the zero-age main sequence is MZAMS ~ 40 M☉. Since the obtained MZAMS is as large as the mass of the collapsing parent cloud, the final stellar mass is likely close to this value. Such massive, rather than exceptionally massive, primordial stars are expected to cause early chemical enrichment of the universe by exploding as black hole-forming super/hypernovae and may also be progenitors of high-redshift γ-ray bursts. The elemental abundance patterns of recently discovered hyper-metal-poor stars suggest that they might have been born from the interstellar medium that was metal-enriched by the supernovae of these massive primordial stars.
Highlights
Recent observations of high redshift galaxies (Kashikawa et al 2006), quasars (Fan et al 2006), and gamma-ray bursts (Totani et al 2006) provide rich information on the ionization structure of the intergalactic medium (IGM) at z ∼ 6 − 7
For the first time, that primordial stars formed from a reionized gas have a smaller mass (∼ 40M⊙) than these first stars
The low temperature of the gas cloud owes to HD cooling, and the correspondingly small collapse mass and low accretion rate lead to the smaller final stellar mass
Summary
Recent observations of high redshift galaxies (Kashikawa et al 2006), quasars (Fan et al 2006), and gamma-ray bursts (Totani et al 2006) provide rich information on the ionization structure of the intergalactic medium (IGM) at z ∼ 6 − 7. The observations indicate that a large fraction of the IGM was already ionized when the Universe was less than one billion years old, but suggest that a substantial fraction was still neutral. Massive stars, are expected to emit ionizing photons for only a few to ten million years, the reionized IGM will start recombining and cooling when the stars die. Earlier (Yoshida et al 2007; hereafter Y07), we used a large cosmological simulation to study the evolution of early relic Hii regions until second-generation gas clouds are form. We compute the gas mass accretion rate and use it as an input to a proto-stellar calculation
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