I will present an overview of some of the exciting results from Spitzer
observations of the young supernova remnant Cassiopeia A. The images from
the MIPS instrument at 24 and 70 microns are dominated by thermal emission
from dust within the remnant and clearly show the counterjet extending in
the opposite direction from the well-known North-East jet. One of the most
surprising results from the 24 micron observations is the presence of
infrared light echoes in the vicinity of Cas A. These are thought to
result from interstellar dust heated by the explosion and by flares from
the compact central object. Images taken in the four IRAC bands show the
presence of synchrotron emission. The spectral indices measured between 6
cm and IRAC channel 1 at 3.6 micron are flatter than those measured between
6 cm and 20 cm indicating a curved synchrotron spectrum consistent with
cosmic-ray-modified shocks. We have also used the IRS instrument in
mapping mode to obtainlow-resolution spectra over nearly the entire extent of Cas A. This is the
largest single spectral map yet constructed with Spitzer. The spectra, in
conjunction with the IRAC images, show that different dust components are
associated with different ejecta/circumstellar media components of the
remnant. The distribution and composition of the inhomogeneous ejecta and
the associated dust indicate that in each direction different
nucleosynthetic layers have reached the reverse shock. We have identified
diffuse Si and S emission near the center of the remnant that matches the
morphology of the absorption seen in the radio at 74 MHz. This material is
thought to be ejecta that has not yet encountered the reverse shock but has
been photoionized by X-ray and ultraviolet radiation from the remnant. We
are able to identify [Ni II] in the remnant. Since the half-life of 56Ni
is only 6 days, this material may not be the remains of the Ni synthesized
in the supernova explosion. Finally, we have identified strong Ne emission
from isolated regions on the northern and southern bright ring. The Ne
emission to the south is red-shifted and has no bright optical or X-ray
counterpart while the Ne emission to the north forms a blue-shifted ring
structure coincident with optical and X-ray oxygen emission. The line
connecting these \"caps\" passes within a few arcseconds of the center of
the remnant, is roughly oriented along the inferred kick direction of the
central compact object, and is approximately at a right angle to the jet
direction, perhaps suggesting a bi-directional outflow of Ne-rich ejecta.
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