CAVITY OF MOLECULAR GAS ASSOCIATED WITH SUPERNOVA REMNANT 3C 397

Abstract

3C 397 is a radio and X-ray bright Galactic supernova remnant (SNR) with an\nunusual rectangular morphology. Our CO observation obtained with the Purple\nMountain Observatory at Delingha reveals that the remnant is well confined in a\ncavity of molecular gas, and embedded at the edge of a molecular cloud (MC) at\nthe local standard of rest systemic velocity of ~32 km/s. The cloud has a\ncolumn density gradient increasing from southeast to northwest, perpendicular\nto the Galactic plane, in agreement with the elongation direction of the\nremnant. This systemic velocity places the cloud and SNR 3C 397 at a kinematic\ndistance of ~10.3 kpc. The derived mean molecular density (~10-30 cm^-3)\nexplains the high volume emission measure of the X-ray emitting gas. A 12CO\nline broadening of the ~32 km/s component is detected at the westmost boundary\nof the remnant, which provides direct evidence of the SNR-MC interaction and\nsuggests multi-component gas there with dense (~10^4 cm^-3) molecular clumps.\nWe confirm the previous detection of a MC at ~38 km/s to the west and south of\nthe SNR and argue, based on HI self-absorption, that the cloud is located in\nthe foreground of the remnant. A list of Galactic SNRs presently known and\nsuggested to be in physical contact with environmental MCs is appended in this\npaper.\n