ar X iv : a st ro - p h / 98 07 01 1 v 1 1 J ul 1 99 8 Submitted to the Astrophysical Journal OH ( 1720 MHz ) Masers As Signposts of Molecular Shocks Dale

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 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 (∆V=30-50 km s) 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 J = 3− 2 column density suggests densities of 10-10 cm. These observations support the hypothesis that the OH(1720 MHz) masers originate in post-shock 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”, which point to the most promising locations to study supernova remnant/molecular cloud interactions. Subject headings: ISM: supernova remnants, molecules – ISM: individual (W28, W44, 3C391) – masers National Radio Astronomy Observatory, Socorro, NM, 87801, USA Department of Astronomy and Physics, Saint Mary’s University, Halifax, NS B3H 3C3, CANADA