Origin of $\alpha$-Poor Very Metal-Poor Stars

Abstract

Among very metal-poor (VMP) stars, $\alpha$-poor VMP ($\alpha$PVMP) stars that have sub-solar values of ${\rm [X/Fe]}$ for Mg and other $\alpha$ elements are rare and are thought to have been formed from gas polluted by Type 1a supernova (SN 1a). However, recent analyses indicate that pure core-collapse supernova (CCSN) ejecta can also be a likely source. We perform a detailed analysis of 17 $\alpha$PVMP stars by considering six different scenarios relevant to the early Galaxy. We consider a single pair-instability supernova (PISN) and a single CCSN. Additionally, we consider the combination of ejecta from a CCSN with ejecta from another CCSN, a PISN, a near-Chandrasekhar mass (near-${\rm M_{Ch}}$) SN 1a, and a sub-Chandrasekhar mass (sub-${\rm M_{Ch}}$) SN 1a. A clear signature can only be established for sub-${\rm M_{Ch}}$ SN 1a with a near-smoking-gun signature in SDSSJ0018-0939 and a reasonably clear signature in ET0381. The majority ($82\%$) of $\alpha$PVMP stars can be explained by pure CCSN ejecta and do not require any SN 1a contribution. However, the combination of CCSN and sub-${\rm M_{Ch}}$ SN 1a ejecta can also explain most ($76\%$) of $\alpha$PVMP stars. In contrast, the combination of ejecta from CCSN with near-${\rm M_{Ch}}$ SN 1a and PISN can fit $41\%$ and $29\%$ of the stars, respectively. The single PISN scenario is strongly ruled out for all stars. Our results indicate that $\alpha$PVMP stars are equally compatible with pure CCSN ejecta and a combination of CCSN and SN 1a ejecta, with sub-${\rm M_{Ch}}$ SN 1a being roughly twice as frequent as near-${\rm M_{Ch}}$ SN 1a.