SDSSJ0018-0939: A Clear Signature of Sub-Chandrasekhar Mass Type 1a Supernova

Abstract

Very metal-poor (VMP) stars ($[Fe/H]\le -2$) that have sub-solar values of $[X/Fe]$ for $\hat{I}\pm$ elements such as Mg, Si, and Ca, are referred to as $\hat{I}\pm$-poor VMP stars. They are quite rare among VMP stars and are thought to have formed from gas enriched predominantly by a single Type Ia supernovae (SN1a) in contrast to most VMP stars which are $\hat{I}\pm$-enhanced and usually associated with core-collapse supernovae. The observed abundance pattern in such stars can provide a direct way to probe the nucleosynthesis in individual SN1a. Although the abundance patterns in some $\hat{I}\pm$-poor VMP stars have been shown to be consistent with SN1a ejecta, a clear nucleosynthetic signature for SN1a resulting from the explosion of a near Chandrasekhar mass (near-${\mathrm{M}}_{\mathrm{Ch}}$) or a sub-Chandrasekhar mass (sub-${\mathrm{M}}_{\mathrm{Ch}}$) white dwarf, has not been unambiguously detected. We perform a detailed analysis of various formation channels of VMP stars and find that the $\hat{I}\pm$-poor VMP star SDSSJ0018-0939, which was earlier reported as a star with potential pair-instability supernova origin, provides almost a smoking-gun signature of a sub-${\mathrm{M}}_{\mathrm{Ch}}$ SN1a resulting from He detonation. We find that compared to other $\hat{I}\pm$-poor VMP stars that were previously identified with SN1a, SDSSJ0018-0939 is the only star that has a clear and unambiguous signature of SN1a. Interestingly, our results are consistent with constraints on SN1a from recent galactic chemical evolution studies that indicate that sub-${\mathrm{M}}_{\mathrm{Ch}}$ SN1a account for $\sim 50–75\%$ of all SN1a and are possibly the dominant channel in the early Galaxy.

14 pages, 4 figures, 2 tables. Submitted to journal