OH (1720 MHz) masers as signposts of molecular shocks

We present observations of molecular gas made with the 15 m James Clark Max well 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 rev eal that the OH masers are preferentially located along the edges of thin f ilaments or clumps of molecular gas. There is a strong correlation between the morphology of the molecular gas and the relativistic gas traced by sync hrotron emission at centimeter wavelengths. Broad CO line widths (Delta V = 30-50 km s(-1)) are seen along these gaseous ridges, while narrow lines ar e seen off the ridges. The ratio of H2CO line strengths is used to determin e temperatures in the broad-line gas of 80 K, and the (CO)-C-13 J = 3-2 col umn density suggests densities of 10(4)-10(5) cm(-3). These observations su pport the hypothesis that the OH (1720 MHz) masers originate in postshock g as, heated by the passage of a supernova remnant shock through dense molecu lar gas. From the observational constraints on the density, velocity, and m agnetic held, we examine the physical properties of the shock and discuss t he shock production of OH. These OH (1720 MHz) masers are useful "signposts " that point to the most promising locations to study supernova remnant/mol ecular cloud interactions.