Mineral formation in stellar winds. IV. Formation of magnesiowüstite

The possible formation of iron-magnesium-oxides in stellar outflows from cool stars with oxygen rich element mixture is discussed. Chemical equilibrium calculations for the non-ideal solid solution magnesiowüstite with composition Mg_xFe_1-x-delta O show that this oxide may be formed under conditions of imperfect equilibrium condensation. A model is developed for calculating the condensation of magnesiowüstite under non equilibrium conditions in stellar outflows and numerical models for multi-component dust condensation including magnesiowüstite in oxygen rich stellar outflows from cool stars are calculated. It is found that magnesiowüstite should be formed in small but probably detectable quantities in outflows from oxygen rich AGB-stars with mass-loss rates dot Mla4 *E^-6 M_sun yr^-1. It forms a feature in the spectrum which seems to correspond to a not yet identified feature seen in some circumstellar dust shells. For higher mass-loss rates condensation of magnesiowüstite is suppressed by early massive condensation of silicates.