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.
