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

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 lambda/Delta lambda of approximately 30 000: the 7(25) - 6(16) line at 29.84 mu m, the 4(41) - 3(12) line at 31.77 mu m, and the 4(32) - 3(03) line at 40.69 mu m One additional emission line, the 4(41) - 4(14) line at 37.98 mu m, and one absorption feature at 38.08 mu m 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 at a resolving power of about 2 000. 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) W cm(-2) respectively, and the equivalent width of the absorption feature was similar to 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. yr(-1) This range lies at least a factor similar to 2 above an independent estimate of the mass-loss rate that may be derived from dynamical considerations, and at least a factor similar to 30 above previous estimates based upon the interpretation of CO observations.