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

We report the astronomical discovery of the 1 10 →1 01 rotational transition within the ν 2 = 1 vibrationally excited state of water vapor (H 2 O). 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 H 2 O masers appear to be as common as SiO masers and H 2 O masers in other transitions, while we fail to detect 658 GHz H 2 O emission toward the W49 N and W51 N star-forming regions. For all of the 11 stars detected, the luminosity in the 658 GHz H 2 O transition is comparable to or higher than the luminosity of any other known SiO or H 2 O maser line.