Astron. Astrophys. 341, 579-586 (1999)

1. Introduction

Asymptotic Giant Branch (AGB) stars are considered to be important contributors of heavy elements to the interstellar medium through their slow but massive stellar winds, and they thereby play an important role in the chemical evolution of galaxies, see e.g. Gustafsson & Ryde (1998). The physics of this mass loss and the processes in winds of AGB stars, such as the nucleation of dust grains and the acceleration of the wind, are, however, not well understood. In trying to gain some insight into the physical and chemical conditions of the wind region and some understanding of the mass-loss mechanism, the Infrared Space Observatory (ISO) has been used to observe the outer atmospheres and the circumstellar regions of a sample of AGB stars by observing spectra of diagnostic molecular bands. As one part in this project we have studied carbon dioxide in the oxygen-rich, semi-regular AGB stars R Crt, R Dor, R Cas, RS CrA, TX Cam, V1111 Oph, and V656 Cas.

A dozen triatomic molecules have been detected in AGB stars to date, mainly at radio wave-lengths (cf. Olofsson et al., 1997). From ground-based telescopes gas-phase CO₂ has been impossible to detect due to the blocking by the terrestrial atmosphere. Only satellite observations provide an opportunity to observe astrophysical CO₂.

The first report on CO₂ emission from M giants was given in Ryde et al. (1997), where it was suggested that the emission feature at in R Crateris was due to the Q-branch rotation-vibration transitions of circumstellar CO₂. Based on more data for six M giants, Justtanont et al. (1998) could prove unequivocally that this feature and others originate from circumstellar gaseous CO₂. On the basis of a preliminary modelling of the emission lines, Justtanont et al. (1998) and Cami et al. (1998) suggest that the emission originates from in a region, in which molecules are formed, which extends a few stellar radii out from the star. From a thermal model they conclude that the gaseous CO₂ has a temperature of more than .

Here, we present new ISO observations of the region of several M giants. CO₂ emission is detected to various degrees in these stars.

© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998
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