Discovery of Interstellar Water Lines at 437, 439, and 471 GHz: Strong Case for Water Maser Formation behind C-Type Shocks

Abstract

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 Caltech Submillimeter Observatory we detected the 439 and 471 GHz lines toward a number of star-forming regions and the 437, 439, and 417 GHz lines 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 ratios of these newly discovered masers, along with the 325 GHz maser line, clearly indicate that the gas temperature within the masing region is >~900 K. This finding is inconsistent with models that predict that H2O masers form behind fast (>= 50 km/s) dissociative shocks. Instead, these new data suggest that slow (<= 50 km/s) nondissociative shocks are a more probable source for the observed maser emission.