1999A&A...345..925T

We present 29-197µm 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, φ∼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 ∼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 {dot}(M)∼3.4x10^–7M_☉/yr and a total ortho and para water vapour abundance (relative to H₂) of f∼1.1x10^–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 29-197µm for the φ∼0.5 phase was 61% of the flux at φ∼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 6x10^–7M_☉/yr previously published for WHya, another oxygen-rich AGB star.