New Neutral–Neutral Pathways in the Chemistry of Silicon- and Sulfur-bearing Molecules in the Cold Interstellar Medium

Abstract Silicon is the most abundant third-row element in space and plays a major role in interstellar dust as silicate particles. Furthermore, silicon is important for the formation of silicate dust in oxygen-rich AGB stars and in supernova remnants. The formation and growth of such particles from the gas phase is a complex subject, believed to be initiated by molecules such as SiO and SiS. In this work we perform high-level electronic structure calculations on the SiSOH system, which allows us to shed light on the reactivity of several silicon and sulfur bearing species, such as SiO, SiS, SO, OH, HSiS, HSO, HSiO, and OSiS. We reveal several new neutral–neutral reaction pathways for formation and destruction that are predicted to occur without a potential energy barrier and thus be possible even in the coldest regions of the interstellar medium. We have found a new neutral–neutral destruction route for SiS (only destruction by atomic oxygen and carbon was previously known) and a possible explanation for the nondetection of OSiS, even though its rotational spectrum is well determined experimentally. Overall, we reveal 14 new relevant reactions and provide an estimate for their rate coefficients, which can be incorporated into astrochemical databases and models of the abundances of these molecules in space.