2021ApJ...921L...7H

Silicon- and oxygen-containing species such as silicon monoxide (SiO) and silicon dioxide (SiO₂) represent basic molecular building blocks connected to the growth of silicate grains in outflows of oxygen-rich asymptotic giant branch (AGB) stars like R Doradus. Yet the fundamental mechanisms of the formation of silicate grains and the early processes that initiate the coupling of the silicon with the oxygen chemistries in circumstellar envelopes have remained obscure. Here, in a crossed molecular beams experiment combined with ab initio electronic structure calculations, we reveal that at least the d2-silaformaldehyde (D₂SiO) and d2-hydroxysilylene (DSiOD) molecules-proxies for the astronomically elusive silaformaldehyde (H₂SiO) and hydroxysilylene (HSiOH) molecules-can be synthesized via the reaction of the D1-silylidyne radical (SiD; X²Π) with D2-water (D₂O) under single-collision conditions. This system represents a benchmark of a previously overlooked class of reactions, in which the silicon-oxygen bond coupling can be initiated by a reaction between the simplest silicon-bearing radical (silylidyne) and one of the most abundant species in the circumstellar envelopes of evolved oxygen-rich AGB stars (water). As supported by novel astrochemical modeling, considering that silicon- and oxygen-containing species like H₂SiO and HSiOH might be photolyzed easily, they ultimately connect to simple molecular precursors such as SiO that drive a chain of reactions conceivably forming higher molecular weight silicon oxides and, ultimately, a population of silicates at high temperatures.