OH 1720 megahertz masers in supernova remnants: C-shock indicators

Recent observations show that the OH 1720 MHz maser is a powerful probe of the shocked region where a supernova remnant strikes a molecular cloud. We perform a thorough study of the pumping of this maser and find tight constraints on the physical conditions needed for its production. The presence of the maser implies moderate temperatures (50-125 K) and densities (similar to 10(5) cm(-3)) and OH column densities of order 10(16) cm(-2). We show that these conditions can exist only if the shocks are of C-type. J-shocks fail by such a wide margin that the presence of this maser could become the most powerful indicator of C-shocks. These conditions also mean that the 1720 MHz maser will be inherently weak compared with the other ground-state OH masers. All the model predictions are in good agreement with the observations.