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

• Published in: The Astrophysical journal Vol. 857; no. 2; pp. 97 - 113
• Main Authors: McWilliam, Andrew, Piro, Anthony L., Badenes, Carles, Bravo, Eduardo
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 20.04.2018; IOP Publishing
• Subjects: Astrophysics; Chemical composition; Copper; Detonation; Diagnostic systems; Dilution; Dwarf galaxies; Ejecta; Galaxies; galaxies: dwarf; Interstellar matter; Interstellar medium; Iron; Manganese; Metallicity; Nickel; Nuclear fusion; nuclear reactions, nucleosynthesis, abundances; Organic chemistry; Progenitors (astrophysics); Stars & galaxies; stars: abundances; Supernova; Supernovae; supernovae: general; White dwarf stars; Zinc
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aab772

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.