Abstract:
The physical process whereby a carbon–oxygen white dwarf explodes as a Type Ia supernova (SN Ia) remains
highly uncertain. The degree of neutronization in SN Ia ejecta holds clues to this process because it depends on the
mass and the metallicity of the stellar progenitor, and on the thermodynamic history prior to the explosion. We
report on a new method to determine ejecta neutronization using Ca and S lines in the X-ray spectra of Type Ia
supernova remnants (SNRs). Applying this method to Suzaku data of Tycho, Kepler, 3C 397, and G337.2−0.7 in
the Milky Way, and N103B in the Large Magellanic Cloud, we find that the neutronization of the ejecta in N103B
is comparable to that of Tycho and Kepler, which suggests that progenitor metallicity is not the only source of
neutronization in SNe Ia. We then use a grid of SN Ia explosion models to infer the metallicities of the stellar
progenitors of our SNRs. The implied metallicities of 3C 397, G337.2−0.7, and N103B are major outliers
compared to the local stellar metallicity distribution functions, indicating that progenitor metallicity can be ruled
out as the origin of neutronization for these SNRs. Although the relationship between ejecta neutronization and
equivalent progenitor metallicity is subject to uncertainties stemming from the 12C + 16O reaction rate, which
affects the Ca/S mass ratio, our main results are not sensitive to these details.
