Formation of Massive Primordial Stars in a Reionized Gas

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.