Iron-rich Metal-poor Stars and the Astrophysics of Thermonuclear Events Observationally Classified as Type Ia Supernovae. I. Establishing the Connection

Abstract

The progenitor systems and explosion mechanisms responsible for the thermonuclear events observationally classified as Type Ia supernovae are uncertain and difficult to uniquely constrain using traditional observations of Type Ia supernova host galaxies, progenitors, light curves, and remnants. For the subset of thermonuclear events that are prolific producers of iron, we use published theoretical nucleosynthetic yields to identify a set of elemental abundance ratios infrequently observed in metal-poor stars but shared across a range of progenitor systems and explosion mechanisms: [Na, Mg, Co/Fe] < 0. We label stars with this abundance signature “iron-rich metal-poor,” or IRMP stars. We suggest that IRMP stars formed in environments dominated by thermonuclear nucleosynthesis and consequently that their elemental abundances can be used to constrain both the progenitor systems and explosion mechanisms responsible for thermonuclear explosions. We identify three IRMP stars in the literature and homogeneously infer their elemental abundances. We find that the elemental abundances of BD +80 245, HE 0533–5340, and SMSS J034249.53–284216.0 are best explained by the (double) detonations of sub-Chandrasekhar-mass CO white dwarfs. If our interpretation of IRMP stars is accurate, then they should be very rare in globular clusters and more common in the Magellanic Clouds and dwarf spheroidal galaxies than in the Milky Way’s halo. We propose that future studies of IRMP stars will quantify the relative occurrences of different thermonuclear event progenitor systems and explosion mechanisms.

Spectacular Nucleosynthesis from Early Massive Stars

“…Each extreme ratio is erased by mixing with ordinary ISM, so invoking multiple element sources requires spatial and temporal coincidence, as the homogenization time in dwarf galaxies is only ∼100-300 Myr (see references in Ji et al 2023). For example, it is tempting to invoke a Type Ia supernova in combination with a massive star supernova to explain the high metallicity, low odd-even, and unusual Fe-peak abundances (in analogy to the "iron-rich metal-poor stars"; Reggiani et al 2023). The C-Ca and neutron-capture elements in J0931+0038 cannot come from the Type Ia supernova, so must instead originate from ISM material mixed with the Type Ia ejecta.…”

“…We performed an extensive search of supernova yield grids covering a wide range of possible nucleosynthesis sites, covering elements from C to Zn: primordial CCSNe exploded with a piston (Heger & Woosley 2010; assuming the S4 location), thermal bomb (Nomoto et al 2013), and kinetic bomb (Limongi & Chieffi 2012) with various assumptions for mixing and fallback; CCSNe of higher metallicities (Nomoto et al 2013;Ritter et al 2018) with rotation (Limongi & Chieffi 2018) and engine-driven explosions (Ebinger et al 2020); HNe of varying energies (Nomoto et al 2013;Grimmett et al 2018); and primordial PISNe (Heger & Woosley 2002;Nomoto et al 2013). We also examined CCSNe of solar metallicity binary stripped stars (Farmer et al 2023) and thermonuclear Type Ia supernovae (references in Reggiani et al 2023), though none of these were good fits, so we do not discuss them further. We only included models with [Z/H] < − 1.5, except for the binary stripped star supernovae, where only solar metallicity models exist.…”