What drives the mass loss of oxygen-rich AGB stars?

New dynamical models for dust-driven winds of oxygen-rich AGB stars are presented which include radiation pressure on dust and molecules, simplified pulsation (piston approximation as inner boundary condition), and 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 heterogeneous grains composed of small islands / inclusions of Mg2SiO4, SiO2, Al2O3, TiO2, and solid Fe. The frequency-dependent treatment of radiative transfer reveals that the gas is cold close to the star (700 - 900 K at 1.5 - 2 R.) which facilitates the nucleation process. The dust temperatures depend strongly on the material composition of the grains, with differences as large as 1000 K, 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 1.5 R-star) and the more opaque Fe-poor Mg-Fe silicates further out. Solid Fe or Fe-contaminated silicates are found to be the only condensates that can efficiently absorb the stellar light in the near IR. Consequently, they play a crucial role for the wind driving mechanism and act as a thermostat close to the star. Only small amounts of Fe can be incorporated into the grains, because otherwise the grains get too hot. Thus, the models reveal almost no mass loss, and no dust shells.