Too little radiation pressure on dust in the winds of oxygen-rich AGB stars

Aims. It is commonly assumed that the massive winds of AGB stars are dust-driven and pulsation-enhanced. However, detailed frequency-dependent dynamical models that can explain the observed magnitudes of mass loss rates and outflow velocities have been published so far only for C-stars. This letter reports on first results of similar models for oxygen-rich AGB stars. The aim is to provide a better understanding of the wind driving mechanism, the dust condensation sequence, and the role of pulsations. Methods. New dynamical models for dust-driven winds of oxygen-rich AGB stars are presented which include frequency-dependent Monte Carlo radiative transfer by means of a sparse opacity distribution technique and a time-dependent treatment of the nucleation, growth and evaporation of inhomogeneous dust grains composed of a mixture of Mg2SiO4, SiO2, Al2O3, TiO2, and solid Fe. Results. The frequency-dependent treatment of radiative transfer reveals that the gas is cold close to the star (700-900K at 1.5-2 R-star) which facilitates the nucleation process. The dust temperatures are strongly material-dependent, with differences as large as 1000 K for different pure materials, which has an important influence on the dust formation sequence. Two dust layers are formed in the dynamical models: almost pure glassy Al2O3 close to the star (r greater than or similar to 1.5R(star)) and the more opaque Fe-poor Mg-Fe-silicates further out. Solid Fe and Fe-rich silicates are found to be the only condensates that can efficiently absorb the stellar light in the near IR. Consequently, they play a key role in the wind driving mechanism and act as a thermostat. Only small amounts of Fe can be incorporated into the grains, because otherwise the grains become too hot. Thus, the models reveal almost no mass loss, and no dust shells. Conclusions. The observed dust sequence Al2O3 -> Fe-poor Mg-Fe-silicates for oxygen-rich AGB stars having low -> high mass loss rates is in agreement with the presented model and can be understood as follows: Al2O3 is present in the extended atmosphere of the star below the wind acceleration region, also without mass loss. The Mg-Fe-silicates form further out and, therefore, their amount depends on the mass loss rate. The driving mechanism of oxygen-rich AGB stars is still an unsolved puzzle.