Condensation of Grains in Circumstellar Shells of AGB Stars

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I review the gas and condensation chemistry of major (e.g., C, N, O, Si, Mg, Fe, Al, Ca, Ti, Cr, Mn, Na, etc.) and trace (notably s-process) elements in circumstellar shells (CSs) of red giant stars as a function of temperature, total pressure (Ptot)), C/O ratio, and metallicity (e.g., [1-4]). Several expected minerals containing the major abundant elements have been identified spectroscopically in CSs and in the laboratory among presolar grains from red giants and AGB stars (e.g., corundum, spinel, SiC, Mg-silicates), but so far, other expected minerals remain elusive in CSs or among presolar grains (e.g., FeNi-metal, FeS, (Ca, Mg, Mn)S, Fe[3]P, NaCl). The redox state of CSs is determined by the C/O ratio; O-rich giants with C/O < 1 produce oxide, silicate, FeS, and metallic dust whereas carbonaceous dust (e.g., graphite, transition metal carbides, SiC), FeSi, FeS, and Mg-silicates form in CSs with C /O > 1. Condensation temperatures (CT) as a function of P(tot) are well known for solar metallicity and for systems with C/O from solar to >1. Generally CTs decrease with decreasing P(tot) at constant metallicity and C/O. The CTs of silicates and oxides drop with increasing bulk C/O ratio, which has interesting consequences for dust formation in S-stars. In contrast CTs of the first major C-bearing condensate (graphite or carbides) increase with increasing bulk C/O, and the C/O in the gas of CSs becomes fixed at ~1. The CTs also depend on stellar metallicity, and generally, a drop in overall metallicity decreases CTs and may change the relative condensation sequence of the different minerals. Both lower absolute condensible mass and lower CTs at lower metallicities will be important for the mass-loss history of AGB stars. Trace elements typically condense in solid solutions with major element host minerals (e.g., rare earths into perovskite, hibonite or SiC) and their abundances in presolar grains will depend on their abundances in stars as well as volatility controlled condensation process. [1] Lodders, K. & Fegley, B. 1995, Meteoritics 30, 661 [2] --1997, AIP Conf. Proc. 402, 391 [3] -- 1999, Proc. IAU Symp. 191, 279 [4] Lodders, K. & Amari S. 2005 Chem Erde, 65, 93.

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