A NEW SUBMILLIMETER WATER MASER TRANSITION AT 325 GHZ

We report the first detection of the 515→422 transition of interstellar water vapor (H2O) near 325 GHz. Using the Caltech Submillimeter Observatory, we detected this line toward the Orion-KL, W49N, W51 Main, and IRAS 1629A star-forming regions. In all cases, maser action is most likely observed. All of the sources with detected 325 GHz emission are also strongly emitting in the well-studied 22 GHz water maser line. Both lines cover similar velocity ranges and, in particular, toward W49N both show high-velocity emission. While the Orion-KL, W49N, and W51 Main maser centers are associated with very luminous infrared and radio continuum sources, IRAS 1629A is a low-luminosity cold IRAS source. It is therefore clear that 325 GHz masers can arise in the same variety of sources as the 22 GHz masers. Unfortunately, absorption by terrestrial water vapor currently permits detection of only the very strongest sources. Our instrumental setup allowed simultaneous observations of the H218O isotopic equivalent of the 515→422 transition near 322 GHz in the image sideband of the receiver. Toward Orion-KL, we tentatively detected emission at the frequency of the H218O line.