  Using the IRAM 30-m telescope, we observed the supernova remnant 3C 391
(G31.9+0.0) and its surroundings in the CO(2-1), HCO+(1-0), CS(2-1), CS(3-2),
and CS(5-4) lines. The ambient molecular gas at the distance (9 kpc) of the
remnant comprises a giant molecular cloud whose edge is closely parallel to a
ridge of bright non-thermal radio continuum, which evidently delineates the
blast-wave into the cloud. We found that in a small (0.6 pc) portion of the
radio shell, the molecular line profiles consist of a narrow (2 km/s)
component, plus a very wide (> 20 km/s) component. Both spectral components
peak within 20" of a previously-detected OH 1720 MHz maser. We name this source
3C 391:BML (broad molecular line); it provides a new laboratory, similar to IC
443 but on a larger scale, to study shock interactions with dense molecular
gas. The wide spectral component is relatively brighter in the
higher-excitation lines. We interpret the wide spectral component as post-shock
gas, either smoothly accelerated or partially dissociated and reformed behind
the shock. The narrow component is either the pre-shock gas or cold gas
reformed behind a fully dissociative shock. Using the 3 observed CS lines, we
measured the temperature, CS column density, and H2 volume density in a dense
clump in the parent molecular cloud as well as the wide-line and narrow-line
portions of the shocked clump. The physical conditions of the narrow-line gas
are comparable to the highest-density clumps in the giant molecular cloud,
while the wide-line gas is both warmer and denser. The mass of compressed gas
in 3C 391:BML is high enough that its self-gravity is significant, and
eventually it could form one or several stars.
