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

We present 29-197 mu m 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, phi similar to 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 similar to 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 (M)over dot similar to 3.4 x 10(-7) M. yr(-1) and a total ortho and para water vapour abundance (relative to H-2) Of f similar to 1.1 x 10(-5). 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 mu m for the phi similar to 0.5 phase was 61% of the flux at phi similar to 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 x 10-7 M. yr(-1) previously published for WHya, another oxygen-rich AGE star.