Schedule Feb 10, 2006
Spectacular Results from Recent Spitzer Observations of Cassiopeia A
Tracey DeLaney (Harvard-Smithsonian CfA)

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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