Early Cosmological H ii /He iii Regions and Their Impact on Second‐Generation Star Formation

Abstract

We present the results of three- dimensional radiation hydrodynamics simulations of the formation and evolution of early H II/ He III regions around the first stars. Cooling and recollapse of the gas in the relic H II region is also followed in a full cosmological context, until second- generation stars are formed. We first carry out ray- tracing simulations of ionizing radiation transfer from the first star. Hydrodynamics is directly coupled with photoionization heating as well as radiative and chemical cooling. The photoionized hot gas is evacuated out of the host halo at a velocity of similar to 30 km s similar to 1. This radiative feedback effect quenches further star formation within the halo for over tens to a hundred million years. We show that the thermal and chemical evolution of the photoionized gas in the relic H II region is remarkably different from that of a neutral primordial gas. Efficientmolecular hydrogen production in the recombining gas enables it to cool to similar to 100 K, where fractionation of HD/ H-2 occurs. The gas further cools by HD line cooling down to a few tens of kelvins. Interestingly, at high redshifts ( z > 10), the minimum gas temperature is limited by that of the cosmic microwave background with T-CMB 2: 728( 1+ z). The gas cloud experiences runaway collapse when its mass is similar to 40M(circle dot) , which is significantly smaller than a typical clump mass of similar to 200 300M(circle dot) for early primordial gas clouds. We argue that massive, rather than very massive, primordial stars may form in the relic H II region. Such stars might be responsible for early metal enrichment of the interstellar medium from which recently discovered hyperYmetal- poor stars were born.