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arXiv · 2609.16826

Testing a Pre-Supernova Contribution to $^{44}$Ti in Cassiopeia A

Toshiki Sato·Joshua Issa·Falk Herwig·Yui Kuboike·J. Martin Laming·Kai Matsunaga·Ryo Sawada
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Abstract

The radioactive isotope $^{44}$Ti in Cassiopeia A has been used as a tracer of the innermost ejecta and explosion asymmetries in core-collapse supernova explosions. However, recent stellar-evolution models suggest substantial $^{44}$Ti may also be synthesized prior to explosion during oxygen-carbon (O-C) shell mergers, a process proposed for the Cas A progenitor. We investigate this pre-supernova contribution by combining NuSTAR measurements of the spatial and kinematic distribution of $^{44}$Ti with X-ray and infrared ejecta diagnostics. The low- and intermediate-velocity $^{44}$Ti components are more closely associated with O-layer tracers than with shocked Fe-rich material. If the $^{44}$Ti associated with the O-layer tracers was synthesized prior to core collapse, it could contribute up to $\sim7\times10^{-5}~M_\odot$, about half of the total. We find that this amount of $^{44}$Ti can be produced in O-C shell-merger models with ingestion rates and convective velocities higher than those in standard one-dimensional stellar-evolution calculations. Stable Fe-group abundance ratios measured with XRISM provide an additional constraint on the explosive contribution, as three-dimensional models with larger $^{44}$Ti yields tend to predict Ti/Fe and Mn/Cr ratios above the observed values. These results suggest that both pre-supernova shell-merger nucleosynthesis and explosive burning contribute to the observed $^{44}$Ti.

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Toshiki Sato, Joshua Issa, Falk Herwig, Yui Kuboike, J. Martin Laming, Kai Matsunaga, Ryo Sawada. 2026-09-15. Testing a Pre-Supernova Contribution to $^{44}$Ti in Cassiopeia A. https://arxiv.org/abs/2609.16826

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