HERSCHEL/HIFI OBSERVATIONS OF A NEW INTERSTELLAR WATER MASER: THE 5₃₂-4₄₁TRANSITION AT 620.701 GHz

Abstract

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared, we have performed mapping observations of the 620.701 GHz 5 32 -4 41 transition of ortho-H 2 O within a ∼1. 5 × 1. 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 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 amplification in all three sources, exhibiting narrow (∼1 km s −1 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 2 yr 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.

The physics of water masers observable with ALMA and SOFIA: model predictions for evolved stars

“…The 620.7-GHz (53,2-44,1) transition was detected as a maser by Neufeld et al (2013) towards Orion KL, Orion S and W49, with clear variability over 2 yr in the last source. The 620.7-GHz line appears to be pumped under physical conditions that are a subset of those that pump 22-GHz masers.…”

Interstellar Hydrides

“…While the shocked region shows intense OH and H 2 O emission lines, the dense and warm environment of the Galactic Center region has a more diverse spectrum with other hydrides like NH 3 In the case of OH, mid-IR transitions with upper state energies, E u /k B of several 10 4 K can also be excited following the photodissociation of H 2 O by shock-generated UV radiation; this phenomenon, observed by Spitzer toward HH 211 and illustrated in Figure 10 Along with the normal (non-inverted) OH and H 2 O line radiation emitted by warm shocked regions, strong maser action is observed in several transitions : these include the 22 GHz 6 16 − 5 23 transition of H 2 O, frequently observed from outflow-driven shock waves along with several submillimeter water maser transitions (e.g. Neufeld et al 2013, and references therein), and the 1720 MHz transition of OH, a signpost of supernova-driven shock waves (e.g Frail & Mitchell 1998). Maser lines are typically characterized by small spots of emission, often unresolved even in long-baseline interferometric observations, narrow line width, and variability on timescales of months or less.…”

Polarisation observations of H2OJK-1 K1= 532− 441620.701 GHz maser emission withHerschel/HIFI in Orion KL

“…Their discovery of this feature gave rise to the present attempt to determine whether the 621 GHz feature might be linearly polarised. In a later search through the Herschel archives Neufeld et al (2013) also found similarly narrow 621 GHz emission features emanating from the Orion south condensation and the W49N region of high-mass star formation. By then, however, the Herschel mission was ending and a search for signs of polarisation in these features was no longer possible.…”

“…We thank D. Neufeld et al (2013) for alerting us to the presence of the 621 GHz transition prior to publication. HIFI has been designed and built by a consortium of institutes and university departments from across …”