CO observations toward supernova remnants with associated OH 1720 MHz masers

The environs of three supernova remnants (SNR) with associated OH 1720 MHz masers, G349.7+0.2, CTB 37A and G16.7+0.1, have been surveyed in the CO J=1-0 transition with the 12 Meter Telescope of the NRAO, using the On-The-Fly technique. These observations have revealed a number of molecular clouds interacting with the SNR shock fronts. Most of the OH 1720 MHz masers have been found to lie over CO concentrations, and the maser velocities are coincident with the CO peak velocities to an accuracy better than 2 km/s. The present data trace the interstellar medium (ISM) structures interacting with the SNRs; however, to probe the shocked molecular gas in which the OH 1720 MHz emission originates, higher excitation transitions and more complex species should be observed. In CTB 37A, where the shock velocity into the molecular cloud could be determined, it has been found to be of C-type, in agreement with theoretical predictions. Part of the rim of G16.7+0.1 appears to be flattened by a dense external cloud, yet the only associated OH 1720 MHz maser lies near the opposite region of the remnant. This behavior, also observed in IC 443 and 3C 391, seems to contradict the suggestion that OH 1720 MHz maser emission occurs mainly for transverse shocks.