Acetylene trimerization on the silicon carbide surface in the envelopes of AGB stars: an astrochemical estimation

This work is devoted to estimating of the contribution of the trimerization reaction of acetylene molecules on the surface of silicon carbide (SiC) particles with the formation of benzene molecules into the benzene abundance in the envelopes of asymptotic giant branch (AGB) stars. The reaction was included into an astrochemical model, using which modeling was carried out under conditions corresponding to the envelope of the AGB star IRC+10216. Based on the modeling results, it is shown that the trimerization reaction of acetylene on the SiC surface can effectively occur under the conditions of the envelopes of AGB stars and have a significant effect on the benzene abundance, and, as a consequence, other aromatic molecules. Accounting for acetylene trimerization can increase the benzene abundance in the gas by an order of magnitude, and at the surface the benzene abundance can be up to four orders of magnitude higher compared to estimates in the gas predicted by a model with only gas-phase reactions. The rate of benzene formation on the SiC surface significantly exceeds the rate of benzene formation in the gas during the early phases of the stellar pulsation. The efficiency of benzene formation in the trimerization reaction depends on currently unknown kinetic parameters of the reaction, in particular, on the desorption energy of the resulting benzene molecule. Determination of reaction parameters will help to perform more accurate quantitative modeling in the future.