The implications of the new Z=0 stellar models and yields on the early metal pollution of the intergalactic medium

Motivated by the recent detection of metals in different components of the high-redshift universe and by the abundance ratios measured in the extremely metal-poor stars of our Galaxy, we study the nucleosynthesis constraints that this imposes on an early generation of stars (Population III). To do so we take into account the chemical yields obtained from homogeneous evolutionary calculations of zero-metal stars in the mass range 3 less than or similar to m/M. less than or similar to 40 (Limongi et al.; Chieffi et al.). We also consider the role played by metal-free very massive objects (m > 100 M.). Using both analytical and numerical chemical evolution models, we confront model predictions from the different choices of the mass function proposed for Population III with the observational constraints. We show that low values of star formation efficiency (< 1%) are required so as not to exceed the minimum metallicity ([C/H] <approximate to> -2.4) measured in the high-redshift systems for any of the initial mass functions (IMFs) proposed. We also show that the observational constraints require Omega (sr) < 3 x 10(-3)<Omega>(b), confirming previous claims that the possible contribution of the stellar remnants from Population III to the baryonic dark matter is insignificant. At present, however, the scarcity of abundance measurements for high-redshift systems does not permit us to put severe limitations on the nature of the initial mass function for Population III. In fact, overabundances of a-elements with respect to iron of the order of those measured in damped Ly alpha systems are obtained for any of the IMFs tested. Nevertheless, to account for the very large [C, N/Fe] ratios found in a considerable number of extremely metal-poor stars of our Galaxy, an IMF peaking at the intermediate stellar mass range (4-8 M.) is needed.