A chemical study of the inner winds of asymptotic giant branch stars

Abstract

Aims. We investigate the non-equilibrium chemistry of the inner winds of AGB stars for different stages of stellar evolution, choosing a standard AGB stellar model and changing photospheric C/O ratios, to describe winds of M, S, and C stars. Chemical formation pathways for several important molecules and the chemistry of S stars and its implications for the nature of the dust forming in these objects are discussed. Methods. The inner wind standard model (≡gas density, temperature, and velocity) is derived from taking into account the effect of shocks induced by stellar pulsation on the gas. The chemistry consists of 68 elements and molecules and 752 chemical reactions. Molecular concentration profiles are derived by solving a system of non-linear, stiff, ordinary, coupled differential equations applied to the wind model gas parameters. Results. We find that unexpected molecules are present in the inner winds, as a result of non-equilibrium chemistry due to shock propagation. In particular, there exists a group of molecules always formed in the inner wind of AGBs, whatever the stage of evolution of the star, i.e., CO, HCN, CS, and SiO, while other groups of species are typical of a O-rich or C-rich chemistry. The shocked regions above the photosphere, where thermal equilibrium does not apply, act as true molecular factories. © ESO 2006.