Discovery of Strong Vibrationally Excited Water Masers at 658 GH[CLC]z[/CLC] toward Evolved Stars

Abstract

We report the astronomical discovery of the 110-->101 rotational transition within the nu 2 = 1 vibrationally excited state of water vapor (H2O). Using the 10.4 m telescope of the Caltech Submillimeter Observatory, we detect strong maser emission in this line, which has a frequency near 658 GHz, toward a diverse sample of oxygen-rich red giant and supergiant stars. In circumstellar envelopes these 658 GHz H2O masers appear to be as common as SiO masers and H2O masers in other transitions, while we fail to detect 658 GHz H2O emission toward the W49 N and W51 N star-forming regions. For all of the 11 stars detected, the luminosity in the 658 GHz H2O transition is comparable to or higher than the luminosity of any other known SiO or H2O maser line.