The effects of Population III stars and variable IMF on the chemical evolution of the Galaxy

We have studied the effects of a hypothetical initial stellar generation (Population III) containing only massive (M > 10M⊙) and very massive stars (M > 100M⊙, Pair-Creation Supernovae) on the chemical evolution of the Milky Way. To this purpose, we have adopted a chemical evolution model – the two-infall model from Chiappini et al. (1997) [Chiappini, C., Matteucci, F., Gratton, R., 1997. ApJ, 477, 765] – which successfully reproduces the main observational features of the Galaxy. Several sets of yields for very massive zero-metallicity stars have been tested: these stars in fact produce quite different amounts of heavy elements, in particular α-elements and iron, than lower mass stars. We have focused our attention on the chemical evolution of α-elements, carbon, nitrogen and iron. It was found that the effects of Population III stars on the Galactic evolution of these elements is negligible if only one or two generations of such stars occurred, whereas they produce quite different results from the standard models if they continuously formed for a longer period. Also the effects of a more strongly variable IMF were discussed and to this purpose we have made use of suggestions appeared in the literature to explain the lack of metal-poor stars in the Galactic halo with respect to model predictions. In these cases the predicted variations in the abundance ratios, the SN rates and the G-dwarf metallicity distribution are more dramatic and always in contrast with observations, so we have concluded that a constant or slightly varying IMF remains the best solution. Our main conclusion is that if very massive stars ever existed they must have formed only for a very short period of time (until the halo gas reached the suggested threshold metallicity of 10−4Z⊙ for the formation of very massive objects); in this case, their effects on the evolution of the elements studied here was negligible also in the early halo phases. In other words, we cannot prove or disprove the existence of such stars on the basis of the available data on very metal poor stars. Because of their large metal production and short lifetimes very massive primordial stars should have enriched the halo gas to the metallicity of the most metal poor stars known ([Fe/H] ∼ −5.4) and beyond in only a few million years. This fact imposes constraints on the number of Pair-Creation Supernovae: we find that a number from 2 to 20 of such SNe occurred in our Galaxy depending on the assumed stellar yields.