Mass and distribution of unshocked iron in Cassiopeia A

Determine the total mass and spatial distribution of unshocked iron ejecta in the supernova remnant Cassiopeia A, given that theoretical models leave the expected amount unclear and existing observations have been unable to derive how much is actually present in the remnant’s interior.

Background

The distribution and yield of elements synthesized in the core-collapse explosion of Cassiopeia A, particularly iron tracing the original 56Ni, are crucial constraints for supernova engine models. While reverse-shocked, Fe-rich ejecta have been mapped in X-rays and associated with a substantial mass, the amount of iron that remains unshocked and resides in the central, photoionized interior of the remnant is not well constrained.

Previous infrared studies (e.g., with Spitzer IRS) could not confidently detect unshocked Fe lines, yielding only upper limits, and deep near-infrared imaging likely traced [Si I] rather than [Fe II]. The authors’ JWST MIRI/MRS observations provide improved sensitivity but still indicate the key unknowns persist, motivating a precise determination of the mass and distribution of unshocked iron in Cas A to inform and validate core-collapse explosion simulations.