Formation of silicate dust around AGB stars

The Helix Planetary Nebula, observed with Spitzer/IRAC

Mass loss from stars is fundamental to the chemical evolution of galaxies. An Asymptotic Giant Branch (AGB) star in the late stages of its evolution can lose a significant fraction of its mass in a short period, during which it is surrounded by an optically thick shell of dust (Gail et al. 2016 A&A 591:A17). This circumstellar dust plays a crucial role in many astrophysical processes: it dominates the energy balance around the star through the absorption and emission of radiation, so that the resulting radiation pressure accelerates the outflow; it is a significant source of interstellar dust particles on which heterogeneous chemical synthesis can occur; and finally the dust is involved in the formation of new planetary systems. In oxygen-rich outflows the dust is observed to be mostly amorphous or crystalline Mg-Fe-silicates. Understanding the initial stages of silicate dust nucleation is the overarching objective of this proposal. We shall: 1) investigate a novel photochemical route for the formation of Mg-Fe-Ca silicates (pyroxenes and olivines), using a combination of ab initio electronic structure calculations and Rice-Ramsperger-Kassel-Markus (RRKM)/master equation rate theory to estimate reaction rate coefficients under relevant conditions of temperature and pressure; 2) investigate the role of SiO in catalysing the formation of highly refractory corundum (Al2O3) nucleation “seeds”; 3) model the subsequent formation in the outflow of large silicate particles, including the effect of periodic shocks and the non-equilibrium chemistry in the shocked gas.

This project is funded by the UK Science and Technology Facilities Council. The investigator is John Plane with project partners Stefan Bromley (IUIC Barcelona) and Leen Decin (U. Leuven; visiting professor at the University of Leeds).