A new submillimeter water maser transition at 325 GHz

The first astronomical detection of the 325 GHz 5(15) - 4(22) transition of interstellar water vapor is reported. The sources toward which the line was detected, namely the luminous IR and radio continuum sources Orion-KL, W49N, and W51 Main and the low-luminosity cold source IRAS 1629A, are also strongly emitting in the 22 GHz water maser line. Both lines show high-velocity emission toward W49N. It is concluded that 325 GHz masers can arise in the same variety of sources as the 22 GHz masers. Emission at the frequency of the H2()-(18) line was tentatively detected toward Orion-KL. 21 refs.