Evidence for a Sub-Chandrasekhar Mass Type~Ia Supernova in the Ursa Minor Dwarf Galaxy

A longstanding problem is identifying the elusive progenitors of Type Ia supernovae (SNe Ia), which can roughly be split into Chandraksekhar and sub-Chandrasekhar mass events. An important difference between these two cases is the nucleosynthetic yield, which is altered by the increased neutron excess in Chandrasekhar progenitors due to their pre-explosion simmering and high central density. From comparison with theoretical nucleosynthesis yields, we show that the chemical composition of the most metal-rich star in the Ursa Minor dwarf galaxy, COS 171, is dominated by nucleosynthesis from a low-metallicity, low-mass, sub-Chandrasekhar mass SN Ia. Key diagnostic abundance ratios include C/Fe, {\alpha}/Fe, Mn/Fe and Ni/Fe ratios, which could not have been produced by Chandrasekhar-mass SNe Ia, Core-Collapse Type II supernovae or Pair-Instsability supernovae. Strong deficiencies of Ni/Fe, Cu/Fe and Zn/Fe also suggest the absence of alpha-rich freeze-out nucleosynthesis, favoring low-mass WD progenitor SNe Ia. Based on comparisons of the measured Mn/Fe and Si/Fe ratios with detonation models, we estimate a WD mass near 0.95Msun. We also compare Mn/Fe and Ni/Fe ratios to the recent theoretical yields predicted by Shen et al., finding consistent results. To explain the COS 171 [Fe/H], at -1.35 dex, requires dilution of the ejecta from a single SNIa event with ~10^4 Msun of material; this is expected for a SN Ia remnant expanding into a warm interstellar medium with n~1 /cm^3. In the future, finding more stars with the unique chemical signatures we highlight here will be important for constraining the rate and environments of sub-Chandrasekhar SNe Ia.