Physico-chemical conditions in circumstellar shells of evolved oxygen-rich M stars

The physical and chemical conditions in circumstellar shells of evolved oxygen-rich stars were studied. Time dependent nonequilibrium chemical calculations for several gas phase species (45 in most models) have been performed to determine their radially dependence abundances. The species are formed from the eight elements H, C, N, O, S, Si, Al, and Cl; 135 chemical reactions are included in the reaction set. Since many reaction rates are temperature dependent, the set of chemical rate equations was solved self-consistently with the radially dependent energy equation. The energy equation included contributions from the cooling of the gas due to expansion, heating from gas-grain collisions, and molecular cooling from H2, CO, and H2O. The circumstellar chemistry is also affected by ultraviolet radiation. All models include the effects of the ambient interstellar field, and the contributions of chromospheric radiation are included in models employing a restricted set of 17 species. It was found that the total column density through the shell determines two important shell characteristics. Higher densities result in large nonadiabatic contributions to the energy equation, and this work shows that for the highest densities studied, the temperature profile is determined by the nonadiabatic terms. Further, the total column density determines the optical depth through the shell in the UV, which controls the position in the shell where photo-induced chemistry can begin