A Nucleosynthetic Origin for the Southwestern Fe-rich Structure in Kepler's Supernova Remnant

{\it Chandra} X-ray observations of Kepler's supernova remnant indicate the existence of a high speed Fe-rich ejecta structure in the southwestern region. We report strong K-shell emission from Fe-peak elements (Cr, Mn, Fe, Ni), as well as Ca, in this Fe-rich structure, implying that those elements could be produced in the inner area of the exploding white dwarf. We found Ca/Fe, Cr/Fe, Mn/Fe and Ni/Fe mass ratios of 1.0--4.1\%, 1.0--4.6\%, 1--11\% and 2--30\%, respectively. In order to constrain the burning regime that could produce this structure, we compared these observed mass ratios with those in 18 one-dimensional Type Ia nucleosynthesis models (including both near-$M_{\rm Ch}$ and sub-$M_{\rm Ch}$ explosion models). The observed mass ratios agree well with those around the middle layer of incomplete Si-burning in Type Ia nucleosynthesis models with a peak temperature of $\sim$(5.0--5.3)$\times$10$^{9}$ K and a high metallicity, Z $>$ 0.0225. Based on our results, we infer the necessity for some mechanism to produce protruding Fe-rich clumps dominated by incomplete Si-burning products during the explosion. We also discuss the future perspectives of X-ray observations of Fe-rich structures in other Type Ia supernova remnants.

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