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We constrain the role of different SN Ia channels in the chemical enrichmentof the Galaxy by studying the abundances of nickel in Galactic stars. Weinvestigate four different SN Ia sub-classes, including the classicalsingle-degenerate near-Chandrasekhar mass SN Ia, the fainter SN Iax systemsassociated with He accretion from the companion, as well as two sub-Ch mass SNIa channels. The latter include the double-detonation of a white dwarfaccreting helium-rich matter and violent white dwarf mergers. NLTE models of Feand Ni are used in the abundance analysis. In the GCE models, we include newdelay time distributions arising from the different SN Ia channels, as well asrecent yields for core-collapse supernovae and AGB stars. The data-modelcomparison is performed using a Markov chain Monte Carlo framework that allowsus to explore the entire parameter space allowed by the diversity of explosionmechanisms and the Galactic SN Ia rate, taking into account the uncertaintiesof the observed data. We show that NLTE effects have a non-negligible impact onthe observed [Ni/Fe] ratios in the Galactic stars. The NLTE corrections to Niabundances are not large, but strictly positive, lifting the [Ni/Fe] ratios by~+0.15 dex at [Fe/H] =-2. We find that that the distributions of [Ni/Fe] in LTEand in NLTE are very tight, with a scatter of < 0.1 dex at all metallicities,supporting earlier work. In LTE, most stars have scaled-solar Ni abundances,[Ni/Fe] = 0, with a slight tendency for sub-solar [Ni/Fe] ratios at lower[Fe/H]. In NLTE, however, we find a mild anti-correlation between [Ni/Fe] andmetallicity, and a slightly elevated [Ni/Fe] ratios at [Fe/H] < -1.0. The NLTEdata can be explained by the GCE models calculated with a substantial, ~ 75%,fraction of sub-Ch SN Ia.

astronomy and astrophysics

Observational constraints on the origin of the elements. V. Non-LTE  abundance ratios of [Ni/Fe] in Galactic stars and enrichment by  sub-Chandrasekhar mass SNe