Abstract
Abstract A long-standing 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. Based on these arguments, 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 Mn/Fe and Ni/Fe, which could not have been produced by a Chandrasekhar-mass SN Ia. Large deficiencies of Ni/Fe, Cu/Fe and Zn/Fe also suggest the absence of alpha-rich freeze-out nucleosynthesis, favoring low-mass white dwarf progenitors of SNe Ia, near 0.95 M ⊙, from comparisons to numerical detonation models. We also compare Mn/Fe and Ni/Fe ratios to the recent yields predicted by Shen et al., finding consistent results. To explain the [Fe/H] at −1.35 dex for COS 171 would require dilution of the SN Ia ejecta with ∼104 M ⊙ of material, which is expected for an SN 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.
Highlights
The low average metallicity of most dwarf galaxies suggests that, like the Milky Way (MW) halo, star formation (SF) was truncated in these systems prior to complete gas consumption, presumably due to gas loss (e.g., Hartwick 1976)
Our abundances are in very good agreement with CH10, including the excitation temperatures derived from Fe I lines, supporting the unusual composition of COS 171 claimed by CH10
We have investigated the highly unusual chemical composition, found by CH10, of star COS 171 in the UMi dwarf galaxy
Summary
The low average metallicity of most dwarf galaxies suggests that, like the Milky Way (MW) halo, star formation (SF) was truncated in these systems prior to complete gas consumption, presumably due to gas loss (e.g., Hartwick 1976). In the Type Ia supernova ( SN Ia) time-delay picture of chemical evolution (e.g., Matteucci & Brocato 1990, MB90), dwarf galaxies experience a low specific star formation rate (SFR), resulting in an increased fraction of nucleosynthesis products from SN Ia versus core-collapse Type II supernovae ( SNe II), compared to the MW at any given metallicity. In this scenario, MB90 predicted that dwarf galaxies would show low [α/Fe]4 ratios compared to the MW disk, due to enhanced iron production from SN Ia without extra α-element synthesis from SN II. In a chemical evolution context, this is most understood as due to a sub-Chandrasekhar-mass SN Ia diluted with ∼104 Me of hydrogen, consistent with expectations for an SNR expanding into a warm interstellar medium
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