Abstract-SOFIA/GREAT Discovery of Terahertz Water Masers

David A. Neufeld Gary J. Melnick, Michael J. Kaufman,  Helmut Wiesemeyer, Rolf Güsten, Alex Kraus4, Karl M. Menten,  Oliver Ricken, and Alexandre Faure, 

http://iopscience.iop.org/article/10.3847/1538-4357/aa7568/meta

We report the discovery of water maser emission at frequencies above 1 THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz  transition of water toward three oxygen-rich evolved stars: W Hya, U Her, and VY CMa. An upper limit on the 1.296 THz line flux was obtained toward R Aql. Near-simultaneous observations of the 22.23508 GHz  water maser transition were carried out toward all four sources using the Effelsberg 100 m telescope. The measured line fluxes imply 22 GHz/1.296 THz photon luminosity ratios of 0.012, 0.12, and 0.83, respectively, for W Hya, U Her, and VY CMa, values that confirm the 22 GHz maser transition to be unsaturated in W Hya and U Her. We also detected the 1.884888 THz transition toward W Hya and VY CMa, and the 1.278266 THz  transition toward VY CMa. Like the 22 GHz maser transition, all three of the THz emission lines detected here originate from the ortho-H₂O spin isomer. Based upon a model for the circumstellar envelope of W Hya, we estimate that stimulated emission is responsible for ~85% of the observed 1.296 THz line emission, and thus that this transition may be properly described as a terahertz-frequency maser. In the case of the 1.885 THz transition, by contrast, our W Hya model indicates that the observed emission is dominated by spontaneous radiative decay, even though a population inversion exists