Detection of far-infrared rotational lines of water vapour toward W Hydrae

David A. Neufeld - Wesley Chen - Gary J. Melnick - Thijs de Graauw - Helmut Feuchtgruber - Leo Haser - Dieter Lutz - Martin Harwit

Abstract:

We report the first detection of thermal water vapour emission from a circumstellar outflow. We have observed four far-infrared rotational emission lines of water vapour and one water absorption feature toward the evolved star W Hydrae, using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Three of the emission lines were observed in the instrument's Fabry-Perot mode at a resolving power of approximately 30000: the
7₂₅ - 6₁₆ line at 29.84 microns, the 4₄₁ - 3₁₂ line at 31.77 microns, and the 4₃₂ - 3₀₃ line at 40.69 microns. One additional emission line, the 4₄₁ - 4₁₄ line at 37.98 microns, and one absorption feature at 38.08 microns that we attribute to a blend of the 13_13,0 - 13_12,1 and the 13_13,1 - 13_12,2 water lines were observed in grating mode ata resolving power of 2000. The observed emission line fluxes were 3.2 x 10^-19, 6.3 x 10^-19, 2.3 x 10^-19 and 2.8 x 10^-19 Wcm^-2 respectively, and the equivalent width of the absorption feature was ~ 10 km s^-1. To within the possible errors in the flux calibration, the observed emission line fluxes can be accounted for simultaneously by a model similar to that of Chen & Neufeld (1995), given a mass-loss rate in the range (0.5 - 3) x 10^-5 M_solar yr^-1 . This range lies at least a factor ~ 2 above an independent estimate of the mass-loss rate that may be derived from dynamical considerations, and at least a factor ~ 30 above previous estimates based upon the interpretation of CO observations.

Keywords:
Stars: circumstellar matter - Stars: late-type - Infrared: stars - Molecular processes

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