2009MNRAS.398.1782R

New observations from the Hubble Ultra Deep Field suggest that the star formation rate at z > 7 drops off faster than previously thought. Using a newly determined star formation rate for the normal mode of Population II/I (PopII/I) stars, including this new constraint, we compute the Thomson scattering optical depth and find a result that is marginally consistent with Wilkinson Microwave Anisotropy Probe 5 results. We also reconsider the role of Population III (PopIII) stars in light of cosmological and stellar evolution constraints. While this input may be needed for reionization, we show that it is essential in order to account for cosmic chemical evolution in the early universe. We investigate the consequences of PopIII stars on the local metallicity distribution function of the Galactic halo (from the recent Hamburg/European Southern Observatory (ESO) survey of metal-poor stars) and on the evolution of abundances with metallicity (based on the ESO large programme on very metal-poor stars), with special emphasis on carbon-enhanced metal-poor stars. The metallicity distribution function shape is well reproduced at low iron abundance ([Fe/H] ≳-4), in agreement with other studies. However, the Hamburg/ESO survey hints at a sharp decrease of the number of low-mass stars at very low iron abundance, which is not reproduced in models with only PopII/I stars. The presence of PopIII stars, of typical masses 30-40 M_☉, helps us to reproduce this feature, leading to a prompt initial enrichment before the onset of PopII/I stars. The metallicity at which this cut-off occurs is sensitive to the lowest mass of the massive PopIII stars, which makes the metallicity distribution function a promising tool to constrain this population. Our most important results show that the nucleosynthetic yields of PopIII stars lead to abundance patterns in agreement with those observed in extremely metal-poor stars. This can be demonstrated by the transition discriminant (a criterion for low-mass star formation taking into account the cooling due to Cii and Oi). In this chemical approach to cosmic evolution, PopIII stars prove to be a compulsory ingredient, and extremely metal-poor stars are inevitably born at high redshift.