Chemical structure of circumstellar shells

Abstract

We discuss nonequilibrium chemistry in isotropic models of expanding circumstellar shells surrounding oxygen-rich red giant stars, using time-scale arguments and chemical kinetic calculations. We predict that most observable molecules should attain significant abundances only beyond a critical radius, approx.3 x 10/sup 14/ cm for low-density shells; beyond that limit H/sub 2/O is efficiently photo-dissociated by interstellar ultraviolet radiation. We show that observation of SiS should give an unambiguous limit to the fraction of circumstellar silicon tied up in grains if chromospheric radiation is unimportant, while SiO is unsuitable for this purpose. Photodissociation of H/sub 2/O by chromospheric radiation may be important in the inner part of the shell, and it is suggested that observations of molecular column densities of H/sub 2/O and other molecules in shells may be used to constrain chromospheric models for red giants.