Acetylene Trimerization on the Silicon Carbide Surface in the Envelopes of AGB Stars: An Astrochemical Estimation

Abstract

This work is devoted to estimating 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, which is used to simulate conditions corresponding to the envelope of the AGB star IRC+10216. The calculation results show that the acetylene trimerization reaction 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. The benzene abundance on the dust surface can be up to four orders of magnitude higher than the estimates for the gas phase predicted by the model with gas-phase reactions only. 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 star’s pulsation. The efficiency of benzene formation in the trimerization reaction and its transition to gas depends on the currently unknown kinetic parameters of the reaction, in particular, on the desorption energy of the resulting benzene molecule. Determining the reaction parameters will help to perform more accurate quantitative modeling in the future.