Spitzer IRAC Observations of Cassiopeia A
ABSTRACT We present Spitzer IRAC images, with representative 5.27 to 38.5 micron IRSspectra, of the Cassiopeia A supernova remnant. Where each IRAC channel isdominant over the others, it illuminates different regions related to thenucleosynthetic layers of the progenitor star, echoing the inhomogeneitiesseen in the X-ray and optical. The Channel 1 (3.19 to 3.94microns) emission mechanism is synchrotron, but spectra towards Channel 1bright patches show a broad featureless continuum peaking around 26microns. We suggest that this is due to un-enriched circumstellar dustfrom 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 astrong, 2-3 micron-wide peak at 21 microns, in addition to ionic lines of[ArII], [ArIII], [SIV] and [NeII], probably indicating where the shock haspenetrated into the oxygen- and silicon-burning layers. Thelong-wavelength continuum emission where Channel 3 (5.02 to 6.44microns) is dominant over the other channels rises gradually to 21microns, with a plateau to longer wavelengths. Channel 2 (4.02 to 5.03microns) depicts, in part, H recombination matching a Paschen beta emissionseen in the image obtained at Palomar, but spectra of strong Channel 2knots show a variety of broadband shapes. Where Channel 2 is very strongcompared to Channel 4, Channel 2 isolates regions where [ArII] is weakcompared to [NeII] in the spectra. In particular, Channels 2 and 3 areconsistent with line and dust emission reflecting only carbon- andneon-burning material. We suggest that all of these findings areconsistent with both the ionic and dust components delineating the distancethe 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 initialforward shock) material that has yet to be re-shocked by the reverse shockthat will heat it to X-ray temperatures. The patchy distribution ofdifferent elements at the reverse shock, in X-rays, optical, andinfrared, would then be due to hydrodynamic inhomogeneities deep in theprogenitor core, as opposed to major azimuthal variations in explosivenucleosynthesis.