Discovery of Interstellar Water Masers at 437, 439, and 471 GHz

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Session 75 -- Star Formation II
Oral presentation, Thursday, 2:30-4:00, Zellerbach Playhouse Room

[75.06] Discovery of Interstellar Water Masers at 437, 439, and 471 GHz

Gary J. Melnick, Karl M. Menten (Harvard-Smithsonian Center for Astrophysics), Thomas G. Phillips (Caltech)

We report the first astronomical detections of the $7_{53}-6_{60}$, $6_{43}-5_{50}$, and the $6_{42}-5_{51}$ transitions of water vapor ($\rm H_2O$) 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 471 GHz lines toward several oxygen-rich evolved stars. Maser action is likely present in all of these lines. The previously detected 325 GHz $5_{15}-4_{22}$ $\rm H_2O$ 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 temperatures within the masing region is $\geq\,800$~K. This finding is inconsistent with the models that predict that masers form behind fast ($\geq 50$~km/s) dissociative shocks. Instead, these new data suggest that slow ($\leq 50$~km/s) nondissociative shocks are a more probable source for the maser emission we observe.

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