Dust Driven Mass Loss in the HRD

Abstract

The upper right corner of the Hertzsprung Russel diagram (HRD) is populated by luminous red giant stars. The most prominent properties of such stars are the presence of small solid particles in their extended atmospheres and the existence of a cool slow massive wind. The dust particles are observable via the pronounced infrared excesses of the objects (e.g. Merril and Stein, 1976). The observation of molecular lines in the radio range shows that the wind is carrying very high mass loss rates (up to 10^-4 M⊙/yr) at wind velocities around 20 km/s (e.g. Knapp and Morris 1985). These mass loss rates are so high that the corresponding time scales (M*/M) are comparable or smaller than the typical time scales for the evolution of the stars on the asymptotic giant branch. Modelling the atmospheres of dust forming red giants is, therefore, not only of interest for a better understanding of the observations of such objects. It is also important for the understanding of the mechanism of wind formation and for a consistent calculation of mass loss rates which are needed for evolutionary calculations.