Astrochemistry and chemistry emulation to simulate dust formation and growth around evolved stars

forthcoming

The chemical evolution of galaxies is driven by mass loss from stars at the end of their lives. The majority of stars will go through an asymptotic giant branch (AGB) phase, losing their outer layers by means of a stellar outflow and enriching the interstellar medium with the building blocks for the next generation of stars and planets. The outflow is triggered by stellar pulsations that aid dust formation, which then launches a dust-driven wind. Large-scale structures, like spirals and disks, are widely observed and thought to be caused by binary interaction with a (sub)stellar companion. Despite the importance of dust to launching the outflow, we still do not know exactly how it is formed. Dust formation is a fundamentally chemical process: gas-phase molecules form larger clusters, condense into a seed particle and grow by accreting more molecules. Existing dust formation models either assume an initial number of seed particles to be already present at the start of the model or use classical nucleation theory, which is not applicable to AGB outflows as it assumes thermal equilibrium. Now, thanks to a decade of quantum chemical calculations, it is finally possible to model dust formation in a chemical kinetic way in AGB outflows. ASHES will develop the first comprehensive chemical network that includes dust nucleation and growth, allowing us to quantify the effects of large-scale structures and a stellar companion’s UV field on the amount, composition, and size of the dust. Using machine learning to emulate the network, we will build the first comprehensive 3D hydrochemical model. This model will quantify the impact of a binary companion on shaping the outflow and, for the first time, on the chemistry and dust formed within the outflow. The observational tracers generated by the model will be used to determine the dusty output of observed outflows. ASHES will transform our understanding of dust formation in AGB outflows and impact AGB research and beyond.