Early cosmic chemical evolution

Abstract

The chemical abundance patterns observed in old and metal-poor stars in the Milky Way predominantly contain the signature of the first supernovae (SNe), and thus allow us to probe the first stars that formed in the universe. Of particular interest is the possible existence of a population of metal-free very massive stars, which are a natural consequence of current theoretical models for primordial star formation at the highest masses. As such, many of these stars would be destined to explode as so-called pair-instability supernovae (PISNe), thus becoming the first sources of metals in the universe. Here, I will argue that the apparent absence of the chemical signature of PISNe in extremely metal-poor (EMP) galactic halo stars may arise from an observational selection effect and should not, by necessity, be taken as an indication that our understanding of primordial star formation is incorrect. Whereas most surveys traditionally focus on the most metal-poor stars, early PISN enrichment is predicted to ‘overshoot’, reaching enrichment levels of [Ca/H]∼- 2.5 that would be missed by current searches. It is further predicted, based on theoretical estimates for the relative number of PISNe, that the expected fraction of stars below [Ca/H]=-2 with a dominant (i.e. >90%) contribution from PISNe is only a few×10-4.