Dust Formation Above Cool Magnetic Spots in Evolved Stars

Abstract

We examine the structure of cool magnetic spots in the photospheres of evolved stars, specifically asymptotic giant branch (AGB) stars and R Coronae Borealis stars. We find that the photosphere of a cool magnetic spot will be above the surrounding photosphere of AGB stars, opposite to the situation in the sun. This results from the behavior of the opacity, which increases with decreasing temperature, opposite to the behavior of the opacity near the effective temperature of the sun. We analyze the formation of dust above the cool magnetic spots, and suggest that the dust formation is facilitated by strong shocks, driven by stellar pulsations, which run through and around the spots. The presence of both the magnetic field and cooler temperatures make dust formation easier as the shock passes above the spot. We review some observations supporting the proposed mechanism, and suggest further observations to check the model.