2025A&A...704A.305D

Context. Dimethyl sulfide (DMS; CH₃ SCH₃) is an organosulfur compound that has been suggested as a potential biosignature in exoplanetary atmospheres. In addition to its tentative detections toward the sub-Neptune planet K2-18b, DMS has been detected in the coma of the 67/P comet and toward the galactic-center molecular cloud G+0.693-0.027. However, its formation routes have not been characterized yet.
Aims. In this work, we aim to investigate five gas-phase reactions (the ion-molecule reactions CH₃SH + CH₃SH₂⁺, CH₃OH + CH₃SH₂⁺, CH₃SH + CH₃OH₂⁺, (CH₃)₂SH⁺ + NH₃, and the CH₃+CH₃S radiative association) in order to characterize DMS formation routes in shocked molecular clouds and star-forming regions.
Methods. We performed dedicated quantum and kinetics calculations to derive the potential energy surfaces of these reactive systems and evaluate the reaction rate coefficients as a function of temperature to be included in astrochemical models.
Results. Among the investigated processes, the reaction between methanethiol (CH₃SH) and protonated methanol (CH₃OH₂⁺), possibly followed by a gentle proton transfer to ammonia, is a compelling candidate to explain the formation of DMS in the galactic-center molecular cloud G+0.693-0.027. The CH₃+CH₃S radiative association does not seem to be a very efficient process, with the exclusion of cold clouds, provided that the thiomethoxy (CH₃ S) and methyl radical are available.
Conclusions. This work does not directly address the potential formation of DMS in the atmospheres of exoplanets. However, it clearly indicates that there are efficient abiotic formation routes of this interesting species. Furthermore, the characterization of the potential energy surface for the CH₃+CH₃ S radiative association supports the recent suggestion that DMS could be formed via photolysis in exoplanetary atmospheres.