The chemical evolution of iron-peak elements with hypernovae

Abstract

We calculate the mean evolution of the iron-peak abundance ratios [(Cr, Mn, Co, Zn)/Fe] in the Galaxy, using modern supernova and hypernova (HN) chemical yields and a Galactic Chemical Evolution code that assumes homogeneous chemical evolution. We investigate a range of HN occurrence rates and are able to produce a chemical composition that is a reasonable fit to the observed values in metal-poor stars. This requires an HN occurence rate that is large (50 percent) in the early Universe, decreasing throughout evolution to a value that is within present-day observational constraints (< 1 per cent). A large HN occurrence rate is beneficial to matching the high [Zn/Fe] observed in the most metal-poor stars, although including HNe with progenitor mass > 60 M0 is detrimental to matching the observed [(Mn, Co)/Fe] evolution at low [Fe/H]. A significant contribution from HNe seems to be critical for producing supersolar [(Co, Zn)/Fe] at low metallicity, though more work will need to be done in order to match the most extreme values. We also emphasize the need to update models for the enrichment sources at higher metallicity, as the satisfactory recovery of the solar values of [(Cr, Mn, Co, Zn)/Fe] still presents a challenge.