CO OBSERVATIONS TOWARD THE SUPERNOVA REMNANT 3C-391

We present observations of CO J = 1-0 emission toward 3C 391, a supern ova remnant whose radio continuum and X-ray emission suggest evolution near a strong density gradient in the surrounding medium. The CO maps reveal that the remnant is located at the edge of a molecular cloud, and the CO emission shows a striking correspondence with the ''breakou t'' morphology of the remnant traced by radio continuum emission. Thes e data support the idea that the progenitor star exploded within a den se molecular cloud and that the supernova blast wave has now broken ou t through the cloud boundary. Bright radio emission coincident with st rong CO emission suggests that the blast wave accelerates electrons to energies of at least 20 GeV, even as it moves into dense neutral mate rial.