Mineral formation in stellar winds. II. Effects of Mg/Si abundance variations on dust composition in AGB stars

Galactic F and G dwarf stars show a considerable star-to-star scatter of their individual abundance ratios of Mg to Si. We study the consequences of such abundance variations on the composition of the dust mixture formed in the circumstellar dust shells if the stars enter the AGB stage of stellar evolution. From thermodynamic equilibrium condensation calculations for the abundant dust components we find: (i) For 1<~epsilon_Mg /epsilon_Si <~ 2 a mixture of forsterite (Mg₂SiO₄) and enstatite (MgSiO₃) is formed which consumes all of the available Mg and Si, (ii) for epsilon_Mg/epsilon_Si <~ 1 enstatite and quartz (SiO₂) are formed, while (iii) for epsilon_Mg /epsilon_Si >~2 forsterite and solid MgO are formed. According to observed Mg/Si abundance ratios, about 90% of all stars fall into the first category, about 10% of all stars seem to fall into the second category, and virtually no stars fall into the third category. For the range of observed Mg/Si abundance ratios 0.6<~epsilon_Mg / epsilon_Si <~ 1.5 we have studied the formation of a multicomponent mixture of dust in circumstellar dust shells by calculating simple stationary wind models including the non-equilibrium condensation of enstatite, forsterite, quartz, solid iron, and solid MgO. The variation of the composition of the non-equilibrium mineral mixture with varying Mg/Si abundance ratio is studied. The mineral mixture found to be formed under non-equilibrium conditions is somewhat different from the chemical equilibrium composition, but follows the general trends found in equilibrium calculations. Particular emphasis is laid on stars with a low Mg/Si abundance ratio (la 1) where quartz grains are shown to form a significant dust component which should also be detectable by the absorption bands of quartz in the IR.