Interaction between the W51C Supernova Remnant and a Molecular Cloud. II. Discovery of Shocked CO and HCO +

Abstract

We report the results of high-resolution H\rI\r21 cm line observations of the shocked interstellar gas in\rthe W51 complex. The shocked H\rI\rgas has been detected between\rand\r]\r180 km s\r~1\r, which\rv\rLSR\r^\r]\r85\ris much greater than the maximum velocity (\rD\r60 km s\r~1\r) permitted by the Galactic rotation toward\rthis direction (\rl\r\\r49¡). The H\rI\rgas is distributed along a looplike Ðlamentary structure of\rD\r20\r@\r]\rD\r3\r@\rsize (or\rD\r35 pc\r]\rD\r5 pc at a distance of 6 kpc). The velocity structure indicates that the H\rI\rgas is a\rportion of an expanding, concave shell. By comparing with the X-ray/CO distributions, we have found\rthat the shocked H\rI\rgas is located at an interface between the X-ray\rÈ\rbright central region of the W51C\rsupernova remnant (SNR) and a large molecular cloud. The correlation between the X-ray, CO, and H\rI\remissions strongly suggests that a shock driven by the SNR is propagating into the molecular cloud. The\rlarge amount (\r[\r1200\rof fast-moving H\rI\rgas indicates that the shock is a fast, radiative, J-type\rM\r_\r)\rshock. The VLA line proÐles yield a maximum\rline\r-\rof\r-\rsight\rshock velocity of\r^\r70 km s\r~1\r. A simple\rmodel in which a hemispherical H\rI\rshell is expanding into a cylindrical cloud from the side could\rexplain the observed morphology and velocity structure of the H\rI\rgas. The shock velocity corrected for\rthe projection is\rD\r100 km s\r~1\r. We derive the shock parameters and discuss the implications for the\rSNR W51C.