Chemical structure of circumstellar shells

Abstract

Nonequilibrium chemistry in isotropic models of expanding circumstellar shells surrounding oxygen-rich red giant stars is discussed, with the use of time-scale arguments and chemical kinetic calculations. It is predicted that most observable molecules should attain significant abundances only beyond a critical radius, approximately 3 x 10 to the 14th cm for low-density shells; beyond that limit H2O is efficiently photo-dissociated by interstellar ultraviolet radiation. It is shown 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 H2O by chromospheric radiation may be important in the inner part of the shell, and it is suggested that observations of molecular column densities of H2O and other molecules in shells may be used to constrain chromospheric models for red giants.