CO rotational line emission from a dense knot in Cas A Evidence for
  active post-reverse-shock chemistry
  We report a Herschel detection of high-J rotational CO lines from a dense
knot in the supernova remnant Cas A. Based on a combined analysis of these
rotational lines, and previously observed ro-vibrational CO lines, we find the
gas to be warm (two components at 400 and 2000 K) and dense (1e6-7 cm-3), with
a CO column density of 5e17 cm-2. This, along with the broad line widths (400
kms-1), suggests that the CO emission originates in the post-shock region of
the reverse shock. As the passage of the reverse shock dissociates any existing
molecules, the CO has most likely reformed in the last few years, in the
post-shock gas. The CO cooling time is comparable to the CO formation time, so
possible heating sources (UV photons from the shock front, X-rays, electron
conduction) to maintain the large column density of warm CO are discussed.
