Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates

摘要: Chemical equilibrium has proven extremely useful to predict the chemical composition of AGB atmospheres. Here we use a recently developed code and an updated thermochemical database, including gaseous condensed species involving 34 elements, compute atmospheres M-, S-, C-type stars. We include for first time TixCy clusters, with x = 1-4 y 1-4, selected larger clusters ranging up Ti13C22, which data is obtained from quantum calculations. find that in general reproduces well observed abundances parent molecules circumstellar envelopes There are however severe discrepancies, various orders magnitude, some molecules: HCN, CS, NH3, SO2 M-type stars, H2O NH3 S-type hydrides H2O, SiH4, PH3 Several not yet atmospheres, like SiC5, SiNH, SiCl, PS, HBO, metal-containing MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, NiS, good candidates detection observatories ALMA. The condensates predicted carbon, TiC, SiC C-rich Al2O3 O-rich outflows. most probable gas-phase precursors dust acetylene, atomic and/or C3 carbon dust, SiC2 Si2C Al AlOH, AlO, Al2O dust. In case TiC Ti probably main supplier titanium. However, predicts Ti8C12 Ti13C22 become major reservoirs titanium at expense region where condensation expected occur, suggesting assembly large could be related formation nuclei TiC.