Winds of M-type AGB stars driven by micron-sized grains

Abstract

Context. In view of the recent problem regarding the dynamical modelling of winds of M-type AGB stars (insufficient radiation pressure on silicate grains), some of the basic assumptions of these models need to be re-evaluated critically. Aims. Accepting the conclusion that non-grey effects will force silicate grains to be virtually Fe-free, the viability of driving winds with micron-sized Fe-free silicates, instead of small particles, is examined. Methods. Using both simple estimates and detailed dynamical atmosphere and wind models, it is demonstrated that radiation pressure on Fe-free silicate grains is sufficient to drive outflows if the restriction to the small particle limit is relaxed, and prevailing thermodynamic conditions allow grains to grow to sizes in the micrometer range. Results. The predicted wind properties, such as mass loss rates and outflow velocities, are in good agreement with observations of M-type AGB stars. Due to a self-regulating feedback between dust condensation and wind acceleration, grain growth naturally comes to a halt at particle diameters of about 1 μm. Conclusions. The most efficient grain sizes to drive winds are in a rather narrow interval around 1 μm. These values are set by the wavelength range corresponding to the flux maximum in typical AGB stars, and are very similar to interstellar grains.