An Alternate Estimate of the Mass of Dust in Cassiopeia A

Recent observations of sub-millimeter continuum emission toward supernova remnants (SNR) have raised the question of whether such emission is caused by dust within the SNR itself or along the line-of-sight. Here we make a comparison of the image of sub-mm emission from dust with the integrated J = 1−0 line emission from interstellar CO toward the SNR Cassiopeia A based on existing data. The cm and mm synchrotron emission from Cas A has a rather symmetric, ring-like structure whereas both the sub-mm continuum and interstellar CO line emission are located mostly toward the south of the SNR. There is positional agreement for 3 of 6 maxima found in CO line and sub-mm continuum emission, with the weakest feature near the center of Cas A and the other two features near the southeast and west edges of the SNR. For these three maxima, a comparison of masses determined from dust and CO data shows good agreement if we use the 450 μm dust absorption coefficient typical for diffuse clouds. There is also good agreement between the sub-mm dust temperature and the gas kinetic temperature from CO and NH3. For the remaining sub-mm continuum peaks, one is outside the forward shock of the SNR. For the other two, one was not mapped in CO; for the other there is no CO emission. Hi absorption covers all of Cas A, but the Hi column density may be too small to account for the sub-mm dust emission. Thus it is possible that one, or perhaps two of these sub-mm continuum peaks are located inside the SNR. From lower resolution maps in CO lines, the SE and W features are the edges of extended clouds. Toward the cloud centers, the CO emission is more intense, but there appears to be less sub-mm dust emission. The differences between CO and sub-mm images may be caused a combination of the techniques used to produce the sub-mm maps and changes in cloud properties from center to edge.