Interaction between the W51C Supernova Remnant and a Molecular Cloud. I. H i 21 Centimeter Line Observations

Abstract

We carried out\r12\rCO\rJ\r\\r1\rÈ\r0,\rJ\r\\r2\rÈ\r1,\r13\rCO\rJ\r\\r1\rÈ\r0, and HCO\r`\rJ\r\\r1\rÈ\r0 line observations of the\rsupernova remnant W51C in order to search for shocked molecules associated with the shocked H\rI\rreported in a companion paper. We discovered shocked CO distributed along an\rD\r8\r@\r(or\rD\r14 pc at a\rdistance of 6 kpc) long arc structure. The position and morphology of the CO distribution coincide with\rthe shocked H\rI\r. The shocked CO has line-of-sight velocities between\r]\r20 and\r]\r50 km s\r~1\rwith\rrespect to the ambient cloud and velocity widths of 8\rÈ\r21 km s\r~1\r. The CO emission is optically thin, and\rK. The shocked CO is clumpy, and we have identiÐed Ðve clumps. Their column densities are\rT\rex\r^\r10\rin the range\rN\r(CO)\r\\r1.2\rÈ\r4.0\r]\r10\r16\rcm\r~2\r. Total mass of the shocked CO is 0.16\rThe HCO\r`\r/CO\rM\r_\r.\rabundance ratio is 1.2\rÈ\r1.3\r]\r10\r~4\r, typical of quiescent molecular clouds. The fast-moving CO and\rHCO\r`\rare probably the molecules that had been destroyed by a fast, dissociative J-type shock and\rre\r-\rformed\rbehind the shock front. We Ðnd that the CO/H\rI\rabundance ratio is generally less than 4\r]\r10\r~5\r,\rexcept for some weak features. The upper limit implies that almost all the hydrogen nuclei in the\rshocked gas is atomic, not molecular. We conclude that the shock is old enough to re-form CO and\rHCO\r`\rmolecules but not old enough to completely re-form\rmolecules. We discuss the variation of\rH\r2\rthe CO content of the shocked gas.