DISCOVERY OF INTERSTELLAR WATER LINES AT 437 GHZ, 439 GHZ, AND 471 GHZ - STRONG CASE FOR WATER MASER FORMATION BEHIND C-TYPE SHOCKS

We report the first astronomical detections of the 7(53) --> 6(60), 6( 43) --> 5(50), and 6(42) --> 5(51) transitions of water vapor (H2O) at frequencies near 437, 439, and 471 GHz, respectively. Using the Calte ch Submillimeter Observatory we detected the 439 and 471 GHz lines tow ard a number of star-forming regions and the 437, 439, and 471 GHz lin es toward several evolved stars. Maser action is likely to be present in all of these lines. The previously detected 325 GHz 5(15) --> 4(22) H2O maser line was also observed toward these sources. Assuming that these transitions are collisionally pumped, as is almost certainly the case for the water masers in star-forming regions, the luminosity rat ios of these newly discovered masers, along with the 325 GHz maser lin e, clearly indicate that the gas temperature within the masing region is greater than or similar to 900 K. This finding is inconsistent with models that predict that H2O masers form behind fast (greater-than-or -equal-to 50 km s-1) dissociative shocks. Instead, these new data sugg est that slow (less-than-or-equal-to 50 km s-1) nondissociative shocks are a more probable source for the observed maser emission.