We report the first detection of thermal water vapour emission from a
circumstellar outflow. We have observed four far-infrared rotational e
mission lines of water vapour and one water absorption feature toward
the evolved star W Hydrae, using the Short Wavelength Spectrometer (SW
S) of the Infrared Space Observatory (ISO). Three of the emission line
s were observed in the instrument's Fabry-Perot mode at a resolving po
wer lambda/Delta lambda of approximately 30 000: the 7(25) - 6(16) lin
e 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 powe
r 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 o
bserved 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 rat
e that may be derived from dynamical considerations, and at least a fa
ctor similar to 30 above previous estimates based upon the interpretat
ion of CO observations.