SpitzerIRAC Images and Sample Spectra of Cassiopeia A’s Explosion

We present Spitzer IRAC images, along with representative 5.27 to 38.5 micronIRS spectra of the Cassiopeia A supernova remnant. We find that various IRACchannels are each most sensitive to a different spectral and physicalcomponent. Channel 1 (3.6 micron) matches radio synchrotron images. WhereChannel 1 is strong with respect to the other channels, the longer-wavelengthspectra show a broad continuum gently peaking around 26 micron, with weak or nolines. We suggest that this is due to un-enriched progenitor circumstellar dustbehind the outer shock, processed by shock photons and electrons. Where Channel4 (8 micron) is bright relative to the other IRAC channels, the long-wavelengthspectra show a strong, 2-3 micron-wide peak at 21 micron, likely due tosilicates and proto-silicates, as well as strong ionic lines of [Ar II], [ArIII], [S IV] and [Ne II]. In these locations, the dust and ionic emissionoriginate from the explosion's O-burning layers. The regions where Channels 2(4.5 micron) and 3 (5.6 micron) are strongest relative to Channel 4 show aspectrum that rises gradually to 21 micron, and then flattens or rises moreslowly to longer wavelengths, along with higher ratios of [Ne II] to [Ar II].Dust and ionic emission in these locations arise primarily from the C- and Ne-burning layers. These findings are consistent with asymmetries in the explosionproducing variations in the velocity structure in different directions, butpreserving the nucleosynthetic layers. At each location, the dust and ioniclines in the mid-infrared, and the hotter and more highly ionized optical andX-ray emission are then dominated by the layer currently encountering thereverse shock in that direction.