Global 3D radiation-hydrodynamical models of AGB stars with dust-driven winds

Abstract

Context. Convection and mass loss by stellar winds are two dynamical processes that shape asymptotic giant branch (AGB) stars and their evolution. Observations and earlier 3D models indicate that giant convection cells cause high-contrast surface intensity patterns, and contribute to the origin of clumpy dust clouds. Aims. We study the formation and resulting properties of dust-driven winds from AGB stars, using new global 3D simulations. Methods. The dynamical stellar interiors, atmospheres, and wind acceleration zones of two M-type AGB stars were modeled with the CO5BOLD code. These first global 3D simulations are based on frequency-dependent gas opacities, and they feature time-dependent condensation and evaporation of silicate grains. Results. Convection and pulsations emerge self-consistently, allowing us to derive wind properties (e.g., mass-loss rates and outflow velocities), without relying on parameterized descriptions of these processes. In contrast to 1D models with purely radial pulsations, the shocks induced by convection and pulsation in the 3D models cover large parts, but not the entirety, of the sphere, leading to a patchy, nonspherical structure of the atmosphere. Since dust condensation critically depends on gas density, new dust clouds form mostly in the dense wakes of atmospheric shocks, where the grains can grow efficiently. The resulting clumpy distribution of newly formed dust leads to a complex 3D morphology of the extended atmosphere and wind-acceleration zone, with simultaneous infall and outflow regions close to the star. Highly nonspherical isotherms and short-lived cool pockets of gas in the stellar vicinity are prominent features. Efficient dust formation sets in closer to the star than spherical averages of the temperature indicate, in dense regions where grain growth rates are higher than average. This can lead to weak outflows in situations where corresponding 1D models do not produce winds. For stars where the overall conditions for dust formation and wind acceleration are favorable, it is unclear whether the resulting mass-loss rates will be higher or lower than in the 1D case. The increased efficiency of dust formation in high-density clumps may be offset by a low volume coverage of the forming clouds. Conclusions. A global 3D approach is essential to make progress in understanding dynamical processes in AGB stars, and, in particular, to solve long-standing problems regarding mass loss.