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

Abstract

Chemical equilibrium has proven extremely useful for predicting the chemical composition of AGB atmospheres. Here we use a recently developed code and an updated thermochemical database that includes 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 Ti x C y clusters, with x = 1-4 and y = 1-4, and selected larger clusters ranging up to Ti 13 C 22 , for which thermochemical data are obtained from quantum-chemical calculations. Our main aims are to systematically survey the main reservoirs of each element in AGB atmospheres, review the successes and failures of chemical equilibrium by comparing it with the latest observational data, identify potentially detectable molecules that have not yet been observed, and diagnose the most likely gas-phase precursors of dust and determine which clusters might act as building blocks of dust grains. We find that in general, chemical equilibrium reproduces the observed abundances of parent molecules in circumstellar envelopes of AGB stars well. There are, however, severe discrepancies of several orders of magnitude for some parent molecules that are observed to be anomalously overabundant with respect to the predictions of chemical equilibrium. These are HCN, CS, NH 3 , and SO 2 in M-type stars, H 2 O and NH 3 in S-type stars, and the hydrides H 2 O, NH 3 , SiH 4 , and PH 3 in C-type stars. Several molecules have not yet been observed in AGB atmospheres but are predicted with nonnegligible abundances and are good candidates for detection with observatories such as ALMA. The most interesting ones are SiC 5 , SiNH, SiCl, PS, HBO, and the metal-containing molecules MgS, CaS, CaOH, CaCl, CaF, ScO, ZrO, VO, FeS, CoH, and NiS. In agreement with previous studies, the first condensates predicted to appear in C-rich atmospheres are found to be carbon, TiC, and SiC, while Al 2 O 3 is the first major condensate expected in O-rich outflows. According to our chemical equilibrium calculations, the gas-phase precursors of carbon dust are probably acetylene, atomic carbon, and/or C 3 , while for silicon carbide dust, the most likely precursors are the molecules SiC 2 and Si 2 C. In the case of titanium carbide dust, atomic Ti is the major reservoir of this element in the inner regions of AGB atmospheres, and therefore it is probably the main supplier of titanium during the formation of TiC dust. However, chemical equilibrium predicts that large titanium-carbon clusters such as Ti 8 C 12 and Ti 13 C 22 become the major reservoirs of titanium at the expense of atomic Ti in the region where condensation of TiC is expected to occur. This suggests that the assembly of large Ti x C y clusters might be related to the formation of the first condensation nuclei of TiC. In the case of Al 2 O 3 dust, chemical equilibrium indicates that atomic Al and the carriers of Al-O bonds AlOH, AlO, and Al 2 O are the most likely gas-phase precursors.

The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars

“…The values of f 0 (SO) range between < 6.8×10 −8 and 6×10 −6 and have a mean fractional abundance of log f 0 (SO) = -6.1 ± 0.6. Chemical equilibrium calculations predict a peak SO abundance in the 1-10 R region of ∼ 10 −7 (Agúndez et al 2020), while nonequilibrium chemical models considering shocks induced by the pulsation of the star predict similar abundances at 5 R (Cherchneff 2006). Therefore, on average, our observed SO abundances are higher than theoretical predictions of the inner wind.…”

“…Thermochemical equilibrium calculations predict that SiO maintains a uniform and high fractional abundance of several 10 −5 from the photosphere out to 10 R in O-rich CSEs, while in C-rich CSEs the predicted fractional abundance from 5 R is also on the order of 10 −5 . The main difference occurs in the innermost region, from the photosphere to around 5 R , where SiO has a low abundance in C-rich conditions while it maintains a high abundance in O-rich CSEs (Agúndez & Cernicharo 2006;Agúndez et al 2020). In a scenario of chemical equilibrium, it seems that the low SiO abundance within 5 R in C-rich CSEs does not have an influence on the final SiO abundance that is injected into the expanding wind, and that chemical equilibrium holds for SiO in the high abundance region located beyond 5 R .…”

“…The mean fractional abundance of SO 2 in the 30 O-rich envelopes studied here is log f 0 (SO 2 ) = -6.2 ± 0.7, that is, very similar to that of SO. Sulfur dioxide is predicted to have low abundances (< 10 −10 ), well below the observed values, in the inner regions of O-rich envelopes according to chemical equilibrium (Agúndez et al 2020). There must be a nonequilibrium process that enhances the formation of SO 2 in the inner envelope.…”

Chemical modelling of dust–gas chemistry within AGB outflows – III. Photoprocessing of the ice and return to the ISM

“…In addition to the O-and C-rich parent species of Papers I and II, we compiled two observationally motivated sets for C-rich outflows. These are based on Agúndez et al (2020), who have compiled (ranges of) observed abundances of parent molecules in C-rich, O-rich, and S-type stars (see their Table 2). One set uses the mean of the ranges in observed abundances, as an approximation of an average C-rich outflow.…”

Detection of infrared fluorescence of carbon dioxide in R Leonis with SOFIA/EXES

“…Most of these molecules have been found in the millimeter range, where molecules without a permanent dipole moment cannot be observed due to their lack of a pure rotational spectrum. One of them is CO 2 , which is predicted to be an abundant parent species by chemical models (e.g., Cherchneff 2006;Agúndez et al 2020) with abundances related to H 2 of 10 −8 −10 −4 . The detections in AGB stars carried out to date have only been done from space with the Infrared Space Observatory (ISO) and the Spitzer Space Telescope (e.g., Justtanont et al 1996Justtanont et al , 1998Ryde et al 1997;Tsuji et al 1997;Yamamura et al 1999;Cami et al 2000;Markwick & Millar 2000;Sloan et al 2010;Smolders et al 2012;Reiter et al 2015; Baylis-Aguirre et al 2020) as a consequence of the extremely high atmospheric opacity.…”