Mass loss mechanisms in evolved stars

Summary Mass loss is a very important phenomenon for stellar evolution. In the late stages of stars it becomes fairly high (up to 10^–4 M/yr) though with a much decreased expansion velocity (about 30 kms^–1). It can also be variable, sporadic. Now, though the first observational evidence for mass loss from cool stars is usually attributed to Adams and McCormak in 1935 and though a lot of observational and theoretical papers have been devoted to it since this discovery, we only know mechanisms which are probably efficient under the dominant physical conditions, but neither one mechanism nor a combination of mechanisms is able to produce the observed effects. Those most invoked (thermal gas pressure, radiation pressure, acoustic waves, shock waves, Alfvén waves, ) will be described and criticized, with emphasis on the radiation pressure on dust grains at work at least in the outer part of cool atmospheres. The geometry and the content of expanding atmospheres are also discussed together with the mechanisms that may be important at small scales. Both theory and observations are taken into accout.