Schedule Feb 10, 2006
Spitzer IRAC Observations of Cassiopeia A
Ennis, Rudnick, Reach, Smith, Rho, DeLaney, Kozasa & Gomez

We present Spitzer IRAC images, with representative 5.27 to 38.5 micron IRS spectra, of the Cassiopeia A supernova remnant. Where each IRAC channel is dominant over the others, it illuminates different regions related to the nucleosynthetic layers of the progenitor star, echoing the inhomogeneities seen in the X-ray and optical. The Channel 1 (3.19 to 3.94 microns) emission mechanism is synchrotron, but spectra towards Channel 1 bright patches show a broad featureless continuum peaking around 26 microns. We suggest that this is due to un-enriched circumstellar dust from the progenitor behind the outer shock and heated by the photons andelectrons from the shock as well as potentially processed (shattered,sputtered) by the shock. Where Channel 4 (6.45 to 9.38 microns) isdominant compared to the other IRAC channels, the spectra show a strong, 2-3 micron-wide peak at 21 microns, in addition to ionic lines of [ArII], [ArIII], [SIV] and [NeII], probably indicating where the shock has penetrated into the oxygen- and silicon-burning layers. Thelong-wavelength continuum emission where Channel 3 (5.02 to 6.44 microns) is dominant over the other channels rises gradually to 21 microns, with a plateau to longer wavelengths. Channel 2 (4.02 to 5.03 microns) depicts, in part, H recombination matching a Paschen beta emission seen in the image obtained at Palomar, but spectra of strong Channel 2 knots show a variety of broadband shapes. Where Channel 2 is very strong compared to Channel 4, Channel 2 isolates regions where [ArII] is weak compared to [NeII] in the spectra. In particular, Channels 2 and 3 are consistent with line and dust emission reflecting only carbon- and neon-burning material. We suggest that all of these findings are consistent with both the ionic and dust components delineating the distance the reverse shockhas penetrated into the nucleosynthetic layers of the ejecta. Thepresence of Si and S in the remnant$apos;s interior, avoiding the Channel 4[ArII] regions, shows the distribution of the once-shocked (by the initial forward shock) material that has yet to be re-shocked by the reverse shock that will heat it to X-ray temperatures. The patchy distribution of different elements at the reverse shock, in X-rays, optical, and infrared, would then be due to hydrodynamic inhomogeneities deep in the progenitor core, as opposed to major azimuthal variations in explosive nucleosynthesis.