Herschel/HIFI observations of a new interstellar water maser: the
  5(32)-4(41) transition at 620.701 GHz
  Using the Herschel Space Observatory's Heterodyne Instrument for the
Far-Infrared (HIFI), we have performed mapping observations of the 620.701 GHz
5(32)-4(41) transition of ortho-H2O within a roughly 1.5 x 1.5 arcmin region
encompassing the Kleinmann-Low nebula in Orion, and pointed observations of
that transition toward the Orion South condensation and the W49N region of
high-mass star formation. Using the Effelsberg 100 m radio telescope, we
obtained ancillary observations of the 22.23508 GHz 6(16)-5(23) water maser
transition; in the case of Orion-KL, the 621 GHz and 22 GHz observations were
carried out within 10 days of each other. The 621 GHz water line emission shows
clear evidence for strong maser amplication in all three sources, exhibiting
narrow (roughly 1 km/s FWHM) emission features that are coincident
(kinematically and/or spatially) with observed 22 GHz features. Moreover, in
the case of W49N - for which observations were available at three epochs
spanning a two year period - the spectra exhibited variability. The observed
621 GHz/22 GHz 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 22 GHz population inversion will be quenched at higher densities than that
of the 621 GHz transition, providing a natural explanation for the
observational fact that 22 GHz maser emission appears to be a necessary but
insufficient condition for 621 GHz maser emission.
