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

Abstract

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.

43 pages, including 13 figures. Accepted for publication in the Astrophysical Journal

References in corpus (11)

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• Survey of Instantaneous 1-22 GHz Spectra of 550 Compact Extragalactic Objects with Declinations from -30deg to +43deg
• Herschel/HIFI observations of O-rich AGB stars : molecular inventory
• The Proper Motion of the Local Group Galaxy IC 10
• Herschel observations of interstellar chloronium
• A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz
• Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H2O and its isotopologues towards Orion KL
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• Polarisation Observations of VY Canis Majoris Water Vapour 5{32}-4{41} 620.701 GHz Maser Emission with HIFI

Cited by in corpus (8)

• Interstellar water chemistry: from laboratory to observations
• Interstellar Hydrides
• The Physics of Water Masers observable with ALMA and SOFIA: Model Predictions for Evolved Stars
• Herschel HIFI observations of O${}_2$ toward Orion: special conditions for shock enhanced emission
• ALMA Observation of the 658 GHz Vibrationally Excited H${}_2$O Maser in Orion KL Source I
• The Science Case for Simultaneous mm-Wavelength Receivers in Radio Astronomy
• H${}_2$O Maser Pumping: The Effect of Quasi-Resonance Energy Transfer in Collisions between H${}_2$ and H${}_2$O Molecules
• Polarisation Observations of H${}_2$O $J_{K_{-1}{K}_1}=5_{32}-4_{41}$ 620.701 GHz Maser Emission with Herschel/HIFI in Orion KL