Abstract-SOFIA/GREAT Discovery of Terahertz Water Masers

David A. Neufeld (JHU), Gary J. Melnick (CfA), Michael J. Kaufman (SJSU), Helmut Wiesemeyer (MPIfR), Rolf Güsten (MPIfR), Alex Kraus (MPIfR), Karl M. Menten (MPIfR), Oliver Ricken (MPIfR),Alexandre Faure (Grenoble)

(Submitted on 26 May 2017)

https://arxiv.org/abs/1705.09672v1

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 8(27)-7(34) 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 6(16)-5(23) water maser transition were carried out towards all four sources using the Effelsberg 100m 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 8(45)-7(53) transition toward W Hya and VY CMa, and the 1.278266 THz 7(43)-6(52) 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