Interaction between the W51C Supernova Remnant and a Molecular Cloud. II. Discovery of Shocked CO and HCO+
We carried out 12CO J = 1-0, J = 2-1, 13CO J = 1-0, and HCO+ J = 1-0 line observations of the supernova remnant W51C in order to search for shocked molecules associated with the shocked H I reported in a companion paper. We discovered shocked CO distributed along an ~8' (or ~14 pc at a distance of 6 kpc) long arc structure. The position and morphology of the CO distribution coincide with the shocked H I. The shocked CO has line-of-sight velocities between +20 and +50 km s-1 with respect to the ambient cloud and velocity widths of 8-21 km s-1. The CO emission is optically thin, and Tex ~= 10 K. The shocked CO is clumpy, and we have identified five clumps. Their column densities are in the range N(CO) = 1.2-4.0 × 1016 cm-2. Total mass of the shocked CO is 0.16 M☉. The HCO+/CO abundance ratio is 1.2-1.3 × 10-4, typical of quiescent molecular clouds. The fast-moving CO and HCO+ are probably the molecules that had been destroyed by a fast, dissociative J-type shock and re-formed behind the shock front. We find that the CO/H I abundance ratio is generally less than 4 × 10-5, except for some weak features. The upper limit implies that almost all the hydrogen nuclei in the shocked gas is atomic, not molecular. We conclude that the shock is old enough to re-form CO and HCO+ molecules but not old enough to completely re-form H2 molecules. We discuss the variation of the CO content of the shocked gas.