2024ApJ...975..108J

Very metal-poor (VMP) stars ([Fe/H] ≤ -2) that have subsolar values of [X/Fe] for α elements such as Mg, Si, and Ca, are referred to as α-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 supernova (SN 1a) in contrast to most VMP stars, which are α-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 SN 1a. Although the abundance patterns in some α-poor VMP stars have been shown to be consistent with SN 1a ejecta, a clear nucleosynthetic signature for SN 1a resulting from the explosion of a near Chandrasekhar mass (near-M_Ch) or a sub-Chandrasekhar mass (sub-M_Ch) white dwarf, has not been unambiguously detected. We perform a detailed analysis of various formation channels of VMP stars and find that the α-poor VMP star SDSS J0018-0939, which was earlier reported as a star with potential pair-instability supernova origin, provides a near-smoking-gun signature of a sub-M_Ch SN 1a resulting from He detonation. We find that compared to other α-poor VMP stars that were previously identified with SN 1a, SDSS J0018-0939 is the only star that has a clear and unambiguous signature of SN 1a. Interestingly, our results are consistent with constraints on SN1 a from recent galactic chemical evolution studies that indicate that sub-M_Ch SN 1a accounts for ∼50%-75% of all SN 1a and is possibly the dominant channel in the early Galaxy.