Nonequilibrium Chemistry in Circumstellar Shells

Abstract

The effects of UV radiation and velocity fields on the chemistry of circumstellar shells around late type stars are investigated. A discussion of the relevant time scales shows that nonequilibrium effects have to be taken into account. In order to demonstrate these non-equilibrium effects, the detailed chemical structure of the shell of α-Ori has been calculated with the physical and chemical parameters taken from CLEGG et al.[1]. The corresponding system of rate equations is solved numerically for a spherically expanding stationary flow. The departure from chemical equilibrium and from kinetic equilibrium is studied both for the wind and the periodic conditions around α-Ori. It is found that, due to UV radiation, most chemical reactions are far from chemical equilibrium, even in the case of a pure photospheric UV field. Photo-ions determine the evolution of the local chemistry. For a constant outflow velocity of 10 km/s, the H₂ concentration shows the most pronounced departure from kinetic equilibrium caused by the specific temperature-dependence of the main H₂-forming reactions and by wind effects. Periodic oscillations of UV radiation and temperature cause a significant departure of the SiO concentration from kinetic equilibrium.