Two populations of metal-free stars in the early Universe

We construct star formation histories at redshifts z ? 5 for two physically distinct populations of primordial, metal-free stars, motivated by theoretical and observational arguments that have hinted towards the existence of an intermediate stellar generation between Population III and Population I/II. Taking into account the cosmological parameters as recently revised by the Wilkinson Microwave Anisotropy Probe after three years of operation, we determine self-consistent reionization histories and discuss the resulting chemical enrichment from these early stellar generations. We find that the bulk of ionizing photons and heavy elements produced at high redshifts must have originated in Population II.5 stars, which formed out of primordial gas in haloes with virial temperatures ?10⁴ K, and had typical masses ?10 M_⊙. Classical Population III stars, formed in minihaloes and having masses ?100 M_⊙, on the other hand, had only a minor impact on reionization and early metal enrichment. Specifically, we conclude that only ?10 per cent by mass of metal-free star formation went into Population III.