<i>SPITZER</i>SPECTRAL MAPPING OF SUPERNOVA REMNANT CASSIOPEIA A

Abstract

We present the global distribution of fine-structure infrared line emission in the Cassiopeia A supernova remnant&#13;\nusing data from the Spitzer Space Telescope’s infrared spectrograph. We identify emission from ejecta materials&#13;\nin the interior, prior to their encounter with the reverse shock, as well as from the postshock bright ring.&#13;\nThe global electron density increases by ≳ 100 at the shock to ~ 10^4 cm^(−3), providing evidence for strong&#13;\nradiative cooling. There is also a dramatic change in ionization state at the shock, with the fading of emission&#13;\nfrom low-ionization interior species like [Si ii] giving way to [S iv] and, at even further distances, highenergy&#13;\nX-rays from hydrogenic silicon. Two compact, crescent-shaped clumps with highly enhanced neon&#13;\nabundance are arranged symmetrically around the central neutron star. These neon crescents are very closely&#13;\naligned with the “kick” direction of the compact object from the remnant’s expansion center, tracing a new&#13;\naxis of explosion asymmetry. They indicate that much of the apparent macroscopic elemental mixing may arise&#13;\nfrom different compositional layers of ejecta now passing through the reverse shock along different directions.