Stellar winds in M-type AGB stars: the role of dust grains and pulsation Venti stellari in stelle AGB di tipo M: ruolo dei grani di polvere e della pulsazione

It is a matter of fact that stars are subject to mass loss events which may be periodic, semi-periodic or episodic and can vary from insignificant to massive. A quantitative description of this phenomenon is necessary to assemble an accurate model for the evolution of stars and galaxies. Such description requires realistic dynamical models that follow the flow of stellar matter from the atmosphere through the wind acceleration zone into the circumstellar region. Three main aspects characterize the mass loss phenomenon in asymptotic giant branch stars: long-period pulsations with the production and propagation of shock-waves through the atmosphere; dust formation in the region where the temperature is low enough for the formation of grains; momentum transfer from the radiation field to the dust particles together with dust-gas and gas-gas collision. In the first chapter I present a short review of the evolution of stars up to the AGB phase and the main events that are typical of that stage of evolution. In the second chapter the topic of dust driven stellar winds is introduced from a general point of view through the analysis of the fundamental mechanisms at the base of wind formation. Finally, in the third chapter I discuss the problem of driving a dusty wind in an M-type star. Oxides and pure silicates have low opacities in the relevant spectral region while dirty silicates, which should be good candidates due to their high opacity, can form only at large distances from the star. I also highlight a recently proposed solution that invokes scattering on large dust grains (with sizes of a few tenths of a micron) and an interesting argument based on the hypothesis that the process of chemisputtering may be negligible in the atmospheres of AGB stars