1996A&A...315L.241B

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 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 6x10^–7M_☉/yr, inner and outer radii for the emitting region of 1.5x10¹⁴ and 1x10¹⁶cm, and a H₂O/H₂ abundance of 8x10^–4 for r ≤4.5x10¹⁴cm and 3x10^–4 at large radii. A decrease of the H₂O/H₂ 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.