Master equation modelling of non-equilibrium chemistry in stellar outflows

Abstract

A current challenge in astrochemistry is to explain the formation of Fe-Mg silicate dust around evolved stars. The dust is observed to form within 2 to 3 stellar radii of oxygen-rich AGB stars, where the typical conditions are kinetic (translational) temperatures between 1200 and 1600 K, and total gas densities below 1011 cm-3. At these high temperatures, molecules with bond energies < 400 kJ mol-1 should be short-lived, and this results in kinetic bottlenecks in postulated mechanisms for converting the observed Fe, Mg, SiO and H2O into silicate. Here we show that, in the very low pressure regime of a stellar outflow, molecules can exhibit significant vibrational disequilibrium because optical transitions - both spontaneous and stimulated by the stellar radiation field - occur on a much faster timescale than collisions. As a result, relatively less stable molecules can form and survive long enough to provide building blocks to silicate formation. Here we use the molecule OSi(OH)2, formed by the recombination of SiO2 and H2O, as an example. When vibrational disequilibrium is accounted for in a master equation treatment which includes optical transitions, the quantity of metal silicates produced in a low mass loss rate evolved star (R Dor) is increased by 6 orders of magnitude.