Early chemical enrichment of the universe and the role of very massive population III stars

Abstract

In this paper the role of very massive population III (pop III) stars in the chemical enrichment of the early universe is discussed. We first compare our predictions with the abundance ratios measured in the high-redshift Lyman-α forest to check whether they are compatible with the values predicted by assuming that the early universe was enriched by massive pop III stars. We conclude that to explain the observed C/Si ratio in the intergalactic medium, a contribution from pop II stars to carbon enrichment is necessary, already at redshift z = 5. We then evaluate the number of pair-instability supernovae (SNγγ) required to enrich the universe to the critical metallicity Zcr, i.e. the metallicity value which causes the transition from a very massive star regime (m > 100 M⊙) to a lower mass regime, similar to the one characteristic of the present time (m < 100M⊙). It is found that between 110 and 115 SNγγ are sufficient to chemically enrich a cubic megaparsec of the intergalactic medium at high redshift for a variety of initial mass functions. The number of ionizing photons provided by these SNγγ and also by the pop III stars ending as black holes was computed and we conclude that there are not enough photons to reionize the universe, being down by at least a factor of ∼3. Finally, we calculate the abundance ratios generated by pop III stars and compare them with the ones observed in low-metallicity damped Lyman-α systems (DLAs). We suggest that pop III stars alone cannot be responsible for the abundance ratios in these objects and that intermediate mass pop II stars must have played an important role especially in enriching DLAs in nitrogen. © 2005 RAS.