Spitzer IRAC images and sample spectra of Cassiopeia A's explosion

We present Spitzer IRAC images and representative 5.27-38.5 mu m IRS spectra of the Cas A SNR. We find that various IRAC channels are each most sensitive to a different spectral and physical component. Channel 1 (3.6 mu m) provides an excellent match to the radio synchrotron images. Where channel 1 is strong with respect to the other IRAC channels, the longer wavelength spectra show a broad continuum gently peaking around 26 mu m, with weak or no lines. We suggest that this is due to unenriched progenitor circumstellar dust behind the outer shock. Where channel 4 (8 mu m) is relatively brightest, the long-wavelength spectra show a strong, 2-3 mu m wide peak at 21 mu m, likely due to silicates and protosilicates. Strong ionic lines of [Ar II], [ Ar III], [S IV], and [Ne II] also appear in these regions. We suggest that in these locations, the dust and ionic emission originate from the explosion's O-burning layers. The regions where channels 2 (4.5 mu m) and 3 (5.6 mu m) are strongest relative to channel 4 show a spectrum that rises gradually to 38 mu m, becoming flatter longward of 21 mu m, along with higher ratios of [Ne II] to [Ar II]. We suggest that the dust and ionic emission in these locations arise primarily from the C- and Ne-burning layers. All of these findings are consistent with asymmetries deep in the explosion, producing variations in the velocity structure in different directions, but generally preserving the nucleosynthetic layering. At each location, the dust and ionic lines in the mid-infrared and the hotter and more highly ionized optical and X-ray emission are then dominated by the layer currently encountering the reverse shock in that direction.