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The physical process whereby a carbon--oxygen white dwarf explodes as a TypeIa supernova (SN Ia) remains highly uncertain. The degree of neutronization inSN Ia ejecta holds clues to this process because it depends on the mass and themetallicity of the stellar progenitor, and on the thermodynamic history priorto the explosion. We report on a new method to determine ejecta neutronizationusing Ca and S lines in the X-ray spectra of Type Ia supernova remnants (SNRs).Applying this method to \textit{Suzaku} data of Tycho, Kepler, 3C 397 andG337.2$-$0.7 in the Milky Way, and N103B in the Large Magellanic Cloud, we findthat the neutronization of the ejecta in N103B is comparable to that of Tychoand Kepler, which suggests that progenitor metallicity is not the only sourceof neutronization in SNe Ia. We then use a grid of SN Ia explosion models toinfer the metallicities of the stellar progenitors of our SNRs. The impliedmetallicities of 3C 397, G337.2$-$0.7, and N103B are major outliers compared tothe local stellar metallicity distribution functions, indicating thatprogenitor metallicity can be ruled out as the origin of neutronization forthese SNRs. Although the relationship between ejecta neutronization andequivalent progenitor metallicity is subject to uncertainties stemming from the$^{12}$C$\,$+$^{16}$O reaction rate, which affects the Ca/S mass ratio, ourmain results are not sensitive to these details.

Observational evidence for high neutronization in supernova remnants: implications for Type Ia supernova progenitors