Spitzerspectral Mapping of Supernova Remnant Cassiopeia A

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