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

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1. Introduction

R Cas is a normal red giant, classified as an M star because its chemical composition is oxygen-rich, with an abundance ratio C/O [FORMULA] 1. It belongs to the Miras, a group of regularly pulsating cool stars. With a period of light variation larger than 1 year ([FORMULA] 430 days), it is classified as a a long-period variable (LPV) star. It pulsates in the fundamental mode while most of LPV stars with periods shorter than 400 days pulsate in an overtone (van Leeuwen et al. 1997).

Mass loss is a fundamental process that drives the star's further evolution on the AGB branch and beyond. Mass loss by AGB stars also dominates the gas and dust injection budgets of the interstellar medium. The mechanism for exciting the Mira emission is not well understood. Bujarrabal et al. (1989) analysed CO (J=1-0) emission from R Cas observed with the IRAM 30m radiotelescope. This single low-excitation millimeter wavelength CO line can partially describe the outer part of the circumstellar envelope. Bujarrabal et al. (1989) estimated a stellar distance of 270 pc, i.e., about 2.5 times the recent Hipparcos measurement, and adopted the envelope characteristics of the carbon star IRC+10216 when deriving the physical parameters for R Cas. Thus, new analyses including observations of other molecules such as water vapour around R Cas are certainly of interest.

Because water vapour is abundant in the terrestrial atmosphere, stellar H2O spectra are often blended with telluric H2O emission/absorption rendering the astronomical lines unobservable, even with high-altitude airborne telescopes. Use of the Infrared Space Observatory (ISO) telescope can avoid this problem, which is inherent to ground-based observations. Infrared (IR) radiation is emitted mainly from the innermost layers of the circumstellar environments. It is in this hot, high-density and strong radiation field region that atoms and molecules are highly excited. Thus, ISO observations would help us to understand the structure of the circumstellar envelope from the inner layers.

We present here the 29-197 µm ISO (LWS-SWS) spectrum of R Cas obtained in three sets of observations. Our circumstellar model used to fit the observed H2O line and continuum fluxes enabled us to derive some physical parameters of the stellar envelope: its mass loss rate, radial kinetic temperature, and radial molecular abundances.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999