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

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 λ/{DELTA}λ of approximately 30000: the 7_25_-6_16_line at 29.84μm, the 4_41_-3_12_line at 31.77μm, and the 4_32_-3_03_ line at 40.69μm. One additional emission line, the 4_41_-4_14_line at 37.98μm, and one absorption feature at 38.08μm 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 2000. The observed emission line fluxes were 3.2x10^-19^, 6.3x10^-19^, 2.3x10^-19^ and 2.8x10^-19^W.cm^-2^ respectively, and the equivalent width of the absorption feature was ~10km.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)x10^-5^M_sun_/yr. 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.