Production of heavy $\alpha$-elements and ${}^{44}$Ti in Cas A: comparison to abundances from 1D core-collapse supernova models and evidence for Carbon-Oxygen shell mergers

Abstract

The merger between the carbon (C) and oxygen (O) shells hours to days before the collapse of a massive star significantly changes its nucleosynthesis, which is reflected in the elemental ratios observed in supernova remnants (SNRs). We present a nucleosynthesis study of ${}^{44}$Ti production in core-collapse supernovae (CCSNe), highlighting large silicon (Si), sulfur (S), calcium (Ca), and, most importantly, argon (Ar) to neon (Ne) ratios as diagnostics for carbon-oxygen (C--O) shell mergers. We compare yields from eight different sets of CCSNe models to observations of Cassiopeia A (Cas A), and show that C--O shell mergers are consistently the models that best match X-ray and infrared observations. These models produce high Ar/Ne ratios ($\gtrsim 0.1$), due to ${}^{20}$Ne depletion and production of ${}^{36}$Ar and ${}^{38}$Ar, while lower ratios are obtained from non-merger cases. Based on the Ar/Ne diagnostic, we compare the range of expected ${}^{44}$Ti produced by C--O shell mergers, which is up to $\sim 20-30\%$ of the overall ${}^{44}$Ti, but expected to be located outside the reverse shock. Based on the sets of models considered, the photon flux expected from the ${}^{44}$Ti synthesized in the C--O shell merger in Cas A is below the $NuSTAR$ and $COSI$ detection limits, compatible with current limits locating most of the ${}^{44}$Ti interior to the reverse shock, but might be detectable from proposed missions like $ASCENT$. Finally, for the SNR of 1987A, a dominant C--O merger origin of the observed ${}^{44}$Ti is unlikely based on the observed redshift in its ${}^{44}$Ti line.

11 pages, 2 figures, submitted

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