论文标题
AGB大气中的化学平衡：小分子，簇和冷凝物的成功，失败和前景
Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
论文作者
论文摘要
化学平衡已被证明在预测AGB大气的化学组成方面非常有用。在这里，我们使用最近开发的代码和更新的热化学数据库，包括涉及34个元素的气态和凝结物种，以计算M-，S-和C型星的AGB大气的化学平衡组成。我们首次包括X = 1-4和Y = 1-4的Tixcy簇，并选择了较大的群集，范围为TI13C22，为此，从量子化学计算中获得了热化学数据。我们发现，在通常的化学平衡中，观察到的AGB恒星中心包膜中父母分子的丰富性很好。但是，对于某些母体分子存在严重的差异，存在各种数量级的差异：在S型星中，M型星，H2O和NH3中的HCN，CS，NH3和SO2，以及C-Type中的Hydrides H2O，H2O，NH3，SIH4和PH3。在AGB大气中尚未观察到的几个分子，例如SIC5，SICH，SICL，PS，HBO，以及含金属的分子MGS MGS，CAS，CAOH，CAOH，CAH，CAH，CAF，CAF，SCO，SCO，ZRO，VO，VO，FOS，FES，FES，COH和NIS，是与Alma这样的夜总会检测的良好候选者。预测的第一个冷凝物是富含C的大气中的碳，抽动和SIC，在O富集流出中的AL2O3。粉尘的最可能的气相前体是乙炔，原子碳和/或C3用于碳灰尘，SIC2和SI2C用于SIC粉尘，以及原子Al和Aloh，Alo，Alo和Al2O，用于Al2O3灰尘。就淡淡的尘埃而言，原子Ti可能是钛的主要供应商。然而，化学平衡预测，Ti8c12和Ti13c22等簇成为钛的主要储层，而预计将发生TIC凝结的地区，以牺牲原子Ti的牺牲，这表明大型TixCy簇的组装可能与第一个冷凝核的形成有关。
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 and condensed species involving 34 elements, to compute the chemical equilibrium composition of AGB atmospheres of M-, S-, and C-type stars. We include for the first time TixCy clusters, with x = 1-4 and y = 1-4, and selected larger clusters ranging up to Ti13C22, for which thermochemical data is obtained from quantum chemical calculations. We find that in general chemical equilibrium reproduces well the observed abundances of parent molecules in circumstellar envelopes of AGB stars. There are however severe discrepancies, of various orders of magnitude, for some parent molecules: HCN, CS, NH3, and SO2 in M-type stars, H2O and NH3 in S-type stars, and the hydrides H2O, NH3, SiH4, and PH3 in C-type stars. Several molecules not yet observed in AGB atmospheres, like SiC5, SiNH, SiCl, PS, HBO, and the metal-containing molecules MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, and NiS, are good candidates for detection with observatories like ALMA. The first condensates predicted are carbon, TiC, and SiC in C-rich atmospheres and Al2O3 in O-rich outflows. The most probable gas-phase precursors of dust are acetylene, atomic carbon, and/or C3 for carbon dust, SiC2 and Si2C for SiC dust, and atomic Al and AlOH, AlO, and Al2O for Al2O3 dust. In the case of TiC dust, atomic Ti is probably the main supplier of titanium. However, chemical equilibrium predicts that clusters like Ti8C12 and Ti13C22 become the major reservoirs of titanium at the expense of atomic Ti in the region where condensation of TiC is expected to occur, suggesting that the assembly of large TixCy clusters could be related to the formation of the first condensation nuclei of TiC.
