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

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 $\sim 7\times 10^{-5}{M}_{\odot }$, 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.

13 pages, 8 figures. Submitted to ApJ