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Using the Herschel Space Observatory's Heterodyne Instrument for theFar-Infrared (HIFI), we have performed mapping observations of the 620.701 GHz5(32)-4(41) transition of ortho-H2O within a roughly 1.5 x 1.5 arcmin regionencompassing the Kleinmann-Low nebula in Orion, and pointed observations ofthat transition toward the Orion South condensation and the W49N region ofhigh-mass star formation. Using the Effelsberg 100 m radio telescope, weobtained ancillary observations of the 22.23508 GHz 6(16)-5(23) water masertransition; in the case of Orion-KL, the 621 GHz and 22 GHz observations werecarried out within 10 days of each other. The 621 GHz water line emission showsclear evidence for strong maser amplication in all three sources, exhibitingnarrow (roughly 1 km/s FWHM) emission features that are coincident(kinematically and/or spatially) with observed 22 GHz features. Moreover, inthe case of W49N - for which observations were available at three epochsspanning a two year period - the spectra exhibited variability. The observed621 GHz/22 GHz line ratios are consistent with a maser pumping model in whichthe population inversions arise from the combined effects of collisionalexcitation and spontaneous radiative decay, and the inferred physicalconditions can plausibly arise in gas heated by either dissociative ornon-dissociative shocks. The collisional excitation model also predicts thatthe 22 GHz population inversion will be quenched at higher densities than thatof the 621 GHz transition, providing a natural explanation for theobservational fact that 22 GHz maser emission appears to be a necessary butinsufficient condition for 621 GHz maser emission.

Herschel/HIFI observations of a new interstellar water maser: the 5(32)-4(41) transition at 620.701 GHz