Interaction between Cassiopeia A and Nearby Molecular Clouds

TL;DR

This study uses infrared and millimeter spectroscopy to investigate the interactions between the supernova remnant Cassiopeia A and surrounding molecular clouds, revealing shock interactions and cloud acceleration by ejecta.

Contribution

It provides new observational evidence of shock-cloud interactions and molecular cloud acceleration caused by supernova ejecta in Cassiopeia A.

Findings

Broadening of CO emission indicates shock interaction with molecular clouds.

High-velocity features suggest acceleration of clouds by supernova ejecta.

Interactions are observed even in clouds slightly away from the remnant's shock front.

Abstract

We present spectroscopy of the supernova remnant Cassiopeia A (Cas A) observed at infrared wavelengths from 10 - 40 micron with the Spitzer Space Telescope and at millimeter wavelengths in 12CO and 13CO J = 2 - 1 (230 and 220 GHz) with the Heinrich Hertz Submillimeter Telescope. The IR spectra demonstrate high-velocity features toward a molecular cloud coincident with a region of bright radio continuum emission along the northern shock front of Cas A. The millimeter observations indicate that CO emission is broadened by a factor of two in some clouds toward Cas A, particularly to the south and west. We believe that these features trace interactions between the Cas A shock front and nearby molecular clouds. In addition, some of the molecular clouds that exhibit broadening in CO lie 1 - 2' away from the furthest extent of the supernova remnant shock front. We propose that this material may be accelerated by ejecta with velocity significantly larger than the observed free-expansion velocity of the Cas A shock front. These observations may trace cloud interactions with fast-moving outflows such as the bipolar outflow along the southwest to northeast axis of the Cas A supernova remnant, as well as fast-moving knots seen emerging in other directions.