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Dust grains forming in the extended atmospheres of AGB stars are critical forthe heavy mass loss of these cool luminous giants, as they provide radiativeacceleration for the stellar winds. Characteristic mid-IR spectral featuresindicate that the grains consist mainly of silicates and corundum. The latterspecies seems to form in a narrow zone within about 2 stellar radii, precedingthe condensation of silicate dust, which triggers the outflow. Recenthigh-angular-resolution observations show clumpy, variable dust clouds at thesedistances. We explore possible causes for the formation of inhomogeneous dustlayers, using 3D dynamical simulations. We modeled the outer convectiveenvelope and the dust-forming atmosphere of an M-type AGB star with the CO5BOLDradiation-hydrodynamics code. The simulations account for frequency-dependentgas opacities, and include a time-dependent description of grain growth andevaporation for corundum and olivine-type silicates. In the inner,gravitationally bound, and corundum-dominated layers of the circumstellarenvelope, a patchy distribution of the dust emerges naturally, due toatmospheric shock waves that are generated by large-scale convective flows andpulsations. The formation of silicate dust at somewhat larger distancesprobably indicates the outer limit of the gravitationally bound layers. Thecurrent models do not describe wind acceleration, but the cloud formationmechanism should also work for stars with outflows. Timescales of atmosphericdynamics and grain growth are similar to observed values. In spherical averagesof dust densities the variable 3D morphology manifests itself as cycle-to-cyclevariations. Grain growth in the wake of large-scale non-spherical shock waves,generated by convection and pulsations, is a likely mechanism for producing theobserved clumpy dust clouds, and for explaining their physical and dynamicalproperties.

Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star