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Description
Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared, we have performed mapping observations of the 620.701GHz 5_32_-4_41_ transition of ortho-H_2_O within a ~1.5'x1.5' region encompassing the Kleinmann-Low nebula in Orion (Orion-KL), and pointed observations of that transition toward the Orion South condensation and the W49N region of high-mass star formation. Using the Effelsberg 100m radio telescope, we obtained ancillary observations of the 22.23508GHz 6_16_-5_23_ water maser transition; in the case of Orion-KL, the 621GHz and 22GHz observations were carried out within 10days of each other. The 621GHz water line emission shows clear evidence for strong maser amplification in all three sources, exhibiting narrow (~1km/s FWHM) emission features that are coincident (kinematically and/or spatially) with observed 22GHz features. Moreover, in the case of W49N --for which observations were available at three epochs spanning a 2yr period-- the spectra exhibited variability. The observed 621GHz/22GHz line ratios are consistent with a maser pumping model in which the population inversions arise from the combined effects of collisional excitation and spontaneous radiative decay, and the inferred physical conditions can plausibly arise in gas heated by either dissociative or non-dissociative shocks. The collisional excitation model also predicts that the 22GHz population inversion will be quenched at higher densities than that of the 621GHz transition, providing a natural explanation for the observational fact that 22GHz maser emission appears to be a necessary but insufficient condition for 621GHz maser emission.
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