Abundances of Sr, Y, and Zr in Metal-Poor Stars and Implications for Chemical Evolution in the Early Galaxy

TL;DR

This paper investigates the origins of certain elements in metal-poor stars, revealing that hypernovae from massive stars are likely responsible for the observed abundances, challenging previous models based on normal supernovae.

Contribution

It introduces a revised model attributing low-metallicity star element abundances to hypernovae, explaining discrepancies with prior supernova-based predictions.

Findings

Normal supernovae cannot account for low Sr, Y, Zr in stars with [Fe/H]<-3

Hypernovae from massive stars are the plausible sources of these elements

Previous models overestimated element abundances in the early Galaxy

Abstract

Studies of nucleosynthesis in neutrino-driven winds from nascent neutron stars show that the elements from Sr through Ag with mass numbers A~88-110 are produced by charged-particle reactions (CPR) during the alpha-process in the winds. Accordingly, we have attributed all these elements in stars of low metallicities ([Fe/H]<-1.5) to low-mass and normal supernovae (SNe) from progenitors of ~8-11M_sun and ~12-25M_sun, respectively, which leave behind neutron stars. Using this rule and attributing all Fe production to normal SNe, we previously developed a phenomenological two-component model, which predicts that [Sr/Fe]>-0.32 for all metal-poor stars. The high-resolution data now available on Sr abundances in Galactic halo stars show that there is a great shortfall of Sr relative to Fe in many stars with [Fe/H]<-3. This is in direct conflict with the above prediction. The same conflict also exists for two other CPR elements Y and Zr. The very low abundances of Sr, Y, and Zr observed in stars with [Fe/H]<-3 thus require a stellar source that cannot be low-mass or normal SNe. We show that this observation requires a stellar source leaving behind black holes and that hypernovae (HNe) from progenitors of ~25-50M_sun are the most plausible candidates. (Abridged)