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

CO rotational line emission from a dense knot in Cas A Evidence for active post-reverse-shock chemistry