Spitzer Spectral Mapping of Supernova Remnant Cassiopeia A

TL;DR

This study maps the infrared emission of Cassiopeia A using Spitzer data, revealing shock interactions, ionization changes, and asymmetric ejecta features linked to explosion dynamics.

Contribution

It provides the first detailed infrared spectral mapping of Cassiopeia A, highlighting shock-induced ionization changes and asymmetric ejecta structures.

Findings

Increased electron density at the shock front (~10^4 cm^-3)

Detection of neon-rich clumps aligned with neutron star motion

Evidence of layered ejecta passing through the reverse shock

Abstract

We present the global distribution of fine structure infrared line emission in the Cassiopeia A supernova remnant using data from the Spitzer Space Telescope's Infrared Spectrograph. We identify emission from ejecta materials in the interior, prior to their encounter with the reverse shock, as well as from the post-shock bright ring. The global electron density increases by >~100 at the shock to ~10^4 cm^-3, providing evidence for strong radiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission from low ionization interior species like [SiII], giving way to [SIV] and, at even further distances, high-energy X-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon abundance are arranged symmetrically around the central neutron star. These neon crescents are very closely aligned with the "kick" direction of the compact object from the remnant's expansion center, tracing a new axis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise from different compositional layers of ejecta now passing through the reverse shock along different directions.