CO rotational line emission from a dense knot in Cassiopeia A

Evidence for active post-reverse-shock chemistry

¹ Department of Earth and Space SciencesChalmers University of Technology, 43992 Onsala, Sweden
e-mail: sofia.wallstrom@chalmers.se
² Department Physik, Universität Basel, 4056 Basel, Switzerland
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
⁵ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
⁶ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁷ SETI Institute, 189 N. Bernardo Ave, Mountain View, CA 94043, USA
⁸ Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, MS 211-3, Moffett Field, CA 94035, USA

Received: 30 August 2013
Accepted: 13 September 2013

Abstract

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 (10⁶⁻⁷ cm^-3), with a CO column density of ~5 × 10¹⁷ cm^-2. This, along with the broad line widths (~400 km s^-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 past several years in the post-shock gas. The CO cooling time is similar to the CO formation time, therefore we discuss possible heating sources (UV photons from the shock front, X-rays, electron conduction) that may maintain the high column density of warm CO.