The rich far-infrared water vapour spectrum of W Hya

We present an ISO Long Wavelength Spectrometer (LWS) grating spectrum of the oxygen-rich AGB star W Hya from 43-197μm. The spectrum is dominated by a forest of water vapour emission lines, confirming that H_2_O molecules are the dominant coolants of the winds of these stars. We have constructed an outflow model for the H_2_O spectrum of W Hya, which successfully matches the fluxes of most of the observed H_2_O lines, using an adopted wind temperature profile. These fits are sensitive to the mass loss rate, to the H_2_O abundance and to the inner radius of the H_2_O emitting region. The best fit parameters correspond to a mass loss rate of 6x10^-7^Msun_/yr, inner and outer radii for the emitting region of 1.5x10^14^ and 1x10^16^cm, and a H_2_O/H_2_ abundance of 8x10^-4^ for r <=4.5x10^14^cm and 3x10^-4^ at large radii. A decrease of the H_2_O/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.