Dust-driven mass loss from RSG stars: global 3D RHD models

The “Great Dimming” of Betelgeuse in 2020 gave unexpected insights, indicating that radiation pressure on dust may play a role for wind formation in RSG stars. Using global 3D RHD models produced with the CO5BOLD code, we investigated causes of dimming events in RSG and AGB stars, and their possible connection with mass loss. We found different mechanisms that can lift dense clouds of material above the top of the convection zone, which then appear as local dark patches when seen against the stellar surface. Apart from periodic dimming due to large-amplitude radial pulsations, two other mechanisms that create obscuring clouds of cool gas in the lower atmosphere were identified: plumes due to Rayleigh-Taylor instabilities, or enhanced convective downdraft and rebound. The latter scenario is much rarer, but it can create a large-scale dark patch that is comparable to the dimming event in Betelgeuse. Our latest 3D RSG star simulations, combining non-equilibrium dust grain growth with an improved treatement of radiative transfer, demonstrate how a localized, sporadic dust-driven outflow can result from such an event.