OH (1720 MHz) masers as signposts of molecular shocks

We present observations of molecular gas made with the 15 m James Clark Maxwell Telescope toward the sites of OH (1720 MHz) masers in three supernova remnants: W28, W44, and 3C 391. Maps made in the (CO)-C-12 J = 3-2 line reveal that the OH masers are preferentially located along the edges of thin filaments or clumps of molecular gas. There is a strong correlation between the morphology of the molecular gas and the relativistic gas traced by synchrotron emission at centimeter wavelengths. Broad CO line widths (Delta V = 30-50 km s(-1)) are seen along these gaseous ridges, while narrow lines are seen off the ridges. The ratio of H2CO line strengths is used to determine temperatures in the broad-line gas of 80 K, and the (CO)-C-13 J = 3-2 column density suggests densities of 10(4)-10(5) cm(-3). These observations support the hypothesis that the OH (1720 MHz) masers originate in postshock gas, heated by the passage of a supernova remnant shock through dense molecular gas. From the observational constraints on the density, velocity, and magnetic field, we examine the physical properties of the shock and discuss the shock production of OH. These OH (1720 MHz) masers are useful "signposts" that point to the most promising locations to study supernova remnant/molecular cloud interactions.