The Rich Far-Infrared Water Vapour Spectrum Of W Hya

We present an ISO Long Wavelength Spectrometer (LWS) grating spectrum of the oxygen-rich AGE star W Hya from 43-197 mu m. The spectrum is dominated by a forest of water vapour emission lines, confirming that H2O molecules are the dominant coolants of the winds of these stars. We have constructed an outflow model for the H2O spectrum of W Hya, which successfully matches the fluxes of most of the observed H2O lines, using an adopted wind temperature profile. These fits are sensitive to the mass loss rate, to the H2O abundance and to the inner radius of the H2O emitting region. The best fit parameters correspond to a mass loss rate of 6 x 10(-7) M. yr(-1) inner and outer radii for the emitting region of 1.5 x 10(14) and 1 x 10(16) cm, and a H2O/H-2 abundance of 8 x 10(-4) for r less than or equal to 4.5 x 10(14) cm and 3 x 10(-4) at large radii. A decrease of the H2O/H-2 abundance in the outer envelope is consistent with the predictions of photochemical models. The availability for the first time of observations of the line fluxes from the dominant coolant species should enable improved models of the wind temperature distribution to be produced.