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

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H2O from R Cas: ISO LWS-SWS observations and detailed modelling *

Truong-Bach 1, R.J. Sylvester 2, M.J. Barlow 2, Nguyen-Q-Rieu 1, T. Lim 3, X.W. Liu 2, J.P. Baluteau 4, S. Deguchi 5, K. Justtanont 6 and A.G.G.M. Tielens 7

1 Observatoire de Paris, DEMIRM and UMR 8540, 61 avenue de l'Observatoire, F-75014 Paris, France
2 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
3 The LWS Instrument-Dedicated Team, ISO Science Operations Center, P.O. Box 50727, E-28080 Madrid, Spain
4 Laboratoire d'Astronomie Spatiale, CNRS, BP 8, F-13376 Marseille Cedex 12, France
5 Nobeyama Radio Observatory, NAO, Minamimaki, Minamisaku, Nagano 384-13, Japan
6 Stockholm Observatory, S-13336 Saltsjöbaden, Sweden
7 Kapteyn Astronomical Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands

Received 10 March 1998 / Accepted 12 March 1999

Abstract

We present 29-197 µm spectra of the oxygen-rich Mira variable star, R Cas, obtained with the Long- and Short- Wavelength Spectrometers (LWS and SWS) on board the Infrared Space Observatory (ISO). The LWS grating observations were made during two pulsational stellar phases, [FORMULA] [FORMULA] 0.5 and 0.2 in August 1996 and June 1997 when the stellar luminosity was near its minimum and mean values, respectively. The infrared flux at the latter epoch was [FORMULA] 30-40% stronger than at the former. SWS grating observations were also made in June 1997. The spectrum presents a strong far-infrared (FIR) continuum and is rich in water lines suitable for use as circumstellar diagnostics.

We have constructed a circumstellar model which consistently treats radiative transfer, chemical exchanges, photodissociation, and heating and cooling effects. The overall FIR excitation field was scaled by a factor which varied with the stellar phase. By fitting the model to the observed FIR water line fluxes and continuum while adopting the stellar parameters based on the Hipparcos distance we have found a mass-loss rate of [FORMULA] [FORMULA] [FORMULA] and a total ortho and para water vapour abundance (relative to "2) of f [FORMULA] 1.1[FORMULA][FORMULA] . The kinetic temperature and the relative abundances of H2O , OH, and O in chemical equilibrium have been derived as functions of radial distance r. H2O excitation is mainly dominated by FIR emitted by dust grains. The deduced model continuum flux at 29-197 µm for the [FORMULA]0.5 phase was 61% of the flux at [FORMULA]0.2. Photodissociation by the FUV interstellar field and CO cooling effects operate farther out than the H2O excitation region. Our derived mass-loss rate of R Cas is similar to the value 6[FORMULA][FORMULA] [FORMULA] previously published for WHya, another oxygen-rich AGB star.

Key words: stars: individual: R Cas – stars: mass-loss – stars: abundances – stars: AGB and post-AGB – infrared: stars – molecular processes

* Based on observations with ISO, an ESA project with instruments funded by ESA Members States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Send offprint requests to: Truong-Bach (tbach@obspm.fr)

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999
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