On the nucleation of dust in oxygen-rich stellar outflows.

Understanding the nature of dust condensation in the outflow from oxygen-rich asymptotic giant branch stars is a continuing problem. A kinetic model has been developed to describe the formation of gas-phase precursors from Ca, Mg, Fe, SiO and TiO in an outflow cooling from 1500 to 1000 K. Electronic structure calculations are used to identify efficient reaction pathways that lead to the formation of metal titanates and silicates. The molecular properties of the stationary points on the relevant potential energy surfaces are then used in a multi-well master equation solver to calculate pertinent rate coefficients. The outflow model couples an explicit treatment of gas-phase chemistry to a volume-conserving particle growth model. CaTiO₃ is shown to be the overwhelming contributor to the formation of condensation nuclei (CN), with less than 0.01 per cent provided by CaSiO₃, (TiO₂)₂ and FeTiO₃. Magnesium species make a negligible contribution. Defining CN as particles with radii greater than 2 nm, the model shows that for stellar mass loss rates above 3×10⁻⁵ M⊙ yr⁻¹, more than 10⁻¹³ CN per H nucleus will be produced when the outflow temperature is still well above 1000 K. This is sufficient to explain the observed number density of grains in circumstellar dust shells.