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Astron. Astrophys. 345, 925-935 (1999)

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5. Conclusion

Within the uncertainties encountered in the observations as discussed, Figs. 1, 2 & 4 and Tables 1-3 show acceptable comparisons between the predicted and observed integrated H2O spectra and FIR continua. The error bars for observed SWS line fluxes are nevertheless high by a factor 2-3; and it is interesting to perform new observations in this wavelength region. Some CO and CII lines are also found in the H2O spectra but their investigation is outside the scope of this paper. Observations I show a spectrum about [FORMULA] 30-40% less intense compared to Observations II's where new or stronger lines appear and some components of a blend become clearly visible. This light variation due to stellar pulsations of the Mira variable between [FORMULA] 300 days corresponds to a phase variation from 0.5, at minimum luminosity, to 0.2 where the luminosity was falling down to its mean value. A best fit to the overall spectra were obtained with the dilution factor for the FIR field scaled by a factor [FORMULA] 0.61 and 1, respectively. This FIR field is mainly dominated by dust. Photodissociation of H2O by the interstellar UV field does not affect significantly the FIR H2O line fluxes, since the effect operates in the outer region ([FORMULA] [FORMULA] cm) where H2O excitation temperatures become weak. The derived H2O abundance and density, the FIR continuum, and the kinetic temperature vary as functions of radius across the circumstellar envelope of R Cas. The derived mass-loss rate is [FORMULA] [FORMULA] 3.4[FORMULA][FORMULA] [FORMULA]  [FORMULA] . The deduced total (ortho and para) H2O abundance is [FORMULA] 1.1[FORMULA][FORMULA] and the kinetic temperature is [FORMULA] [FORMULA] 1300 K at the inner radius of the envelope, and they decrease with distance from the star. Compared to W Hya, another O-rich AGB star, the initial H2O abundance of R Cas is about one order of magnitude lower while the mass-loss rate is nearly the same; the values found for W Hya (1.2[FORMULA][FORMULA] and 6[FORMULA][FORMULA] [FORMULA]  [FORMULA] ; see Barlow et al. 1996) were determined, however, by using a different model. Similar mass-loss rates ([FORMULA] [FORMULA]) have been derived for a number of LPV Miras using CO radio observations (e.g. Knapp et al. 1982, Loup et al. 1993, Kahane & Jura 1994, Gonzalez-Alfonso et al. 1998), and optical KI /NaI scattering (e.g. Guilain & Mauron 1996).

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999
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