The effects of the initial mass function on Galactic chemical enrichment

Abstract

Context. We have been seeing mounting evidence that the stellar initial mass function (IMF) might extend far beyond the canonical Mi ∼ 100 M⊙ limit, but the impact of such a hypothesis on the chemical enrichment of galaxies is yet to be clarified. Aims. We aim to address this question by analysing the observed abundances of thin- and thick-disc stars in the Milky Way with chemical evolution models that account for the contribution of very massive stars dying as pair instability supernovae. Methods. We built new sets of chemical yields from massive and very massive stars up to Mi ∼ 350 M⊙ by combining the wind ejecta extracted from our hydrostatic stellar evolution models with explosion ejecta from the literature. Using a simple chemical evolution code, we analysed the effects of adopting different yield tables by comparing predictions against observations of stars in the solar vicinity. Results. After several tests, we set our focus on the [O/Fe] ratio that best separates the chemical patterns of the two Milky Way components. We find that with a standard IMF, truncated at Mi ∼ 100 M⊙, we can reproduce various observational constraints for thin-disc stars; however, the same IMF fails to account for the [O/Fe] ratios of thick-disc stars. The best results are obtained by extending the IMF up to Mi = 350 M⊙, while including the chemical ejecta of very massive stars in the form of winds and pair instability supernova (PISN) explosions. Conclusions. Our study indicates that PISN may have played a significant role in shaping the chemical evolution of the thick disc of the Milky Way. Including their chemical yields makes it easier to reproduce not only the level of the α-enhancement, but also the observed slope of thick-disc stars in the [O/Fe] vs. [Fe/H] diagram. The bottom line is that the contribution of very massive stars to the chemical enrichment of galaxies is potentially quite important and should not be neglected in models of chemical evolution.