Interstellar and Ejecta Dust in the Cas A Supernova Remnant
  Infrared continuum observations provide a means of investigating the physical
composition of the dust in the ejecta and swept up medium of the Cas A
supernova remnant. Using low resolution Spitzer IRS spectra (5-35 $\mu$m), and
broad-band Herschel PACS imaging (70, 100, and 160 $\mu$m), we identify
characteristic dust spectra, associated with ejecta layers that underwent
distinct nuclear burning histories. The most luminous spectrum exhibits strong
emission features at $\sim9$ and 21 $\mu$m and is closely associated with
ejecta knots with strong Ar emission lines. The dust features can be reproduced
by magnesium silicate grains with relatively low Mg to Si ratios. Another dust
spectrum is associated with ejecta having strong Ne emission lines. It has no
indication of any silicate features, and is best fit by Al$_2$O$_3$ dust. A
third characteristic dust spectrum shows features that are best matched by
magnesium silicates with a relatively high Mg to Si ratio. This dust is
primarily associated with the X-ray emitting shocked ejecta, but it is also
evident in regions where shocked interstellar or circumstellar material is
expected. However, the identification of dust composition is not unique, and
each spectrum includes an additional featureless dust component of unknown
composition. Colder dust of indeterminate composition is associated with
emission from the interior of the SNR, where the reverse shock has not yet
swept up and heated the ejecta. Most of the dust mass in Cas A is associated
with this unidentified cold component, which is $\lesssim0.1$ $M_{\odot}$. The
mass of warmer dust is only $\sim 0.04$ $M_{\odot}$.
