Condensation Chemistry of Circumstellar Grains

Thermochemical equilibrium calculations are successful in predicting the mineralogy as well as the major and trace element chemistry of circumstellar grains found in meteorites. The calculations also explain observations of dust close to AGB stars (within 1–3 stellar radii). The trace element chemistry in circumstellar graphite, SiC, and other refractory carbide grains agrees with equilibrium condensation calculations for circumstellar shells of carbon stars. Observed trace element abundance patterns in N stars are complementary to those found in SiC grains indicating fractional condensation in circumstellar shells. Condensation temperatures depend upon total pressure, C/O ratio, nitrogen abundances, and overall metallicity. Therefore for condensation temperatures to be meaningful, the total pressure and elemental abundances (i.e., C/O ratio, metallicity) must be specified.