Condensation of MgS in outflows from carbon stars

Aims. The basic mechanism responsible for the widespread condensation of MgS in the outflows from carbon-rich stars on the tip of the AGB is discussed with the aim of developing a condensation model that can be applied in model calculations of dust formation in stellar winds. Methods. The different possibilities for how MgS may be formed in the chemical environment of outflows from carbon stars are explored by some thermochemical calculations and by a detailed analysis of the growth kinetics of grains in stellar winds. The optical properties of core-mantle grains with an MgS mantle are calculated to demonstrate that such grains reproduce the structure of the observed 30 mu m feature. These considerations are complemented by model calculations of circumstellar dust shells around carbon stars. Results. It is argued that MgS is formed via precipitation on silicon carbide grains. This formation mechanism explains some of the basic observed features of MgS condensation in dust shells around carbon stars. A weak secondary peak at about 33... 36 mu m is shown to exist in certain cases if MgS forms a coating on SiC. This new feature seems to have occasionally been observed.