The Important Role of Hydrogen in the Formation of Silicon Carbide in Evolved Stars

Cosmic dust mainly forms in the atmospheres of evolved stars. In carbon-rich stars, amorphous carbon along with silicon carbide are the main constituents of dust grains, yet the mechanisms involved in the formation of these grains are poorly understood. Several molecular precursors have been proposed to form silicon carbide grains. Here we have simulated in the laboratory the formation of silicon carbide dust starting from atomic C, atomic Si and H2, and we have clearly identified SiC2 as a key molecular precursor of nanodust analogues. We show that the interaction of molecular hydrogen with atomic carbon initiates the formation of hydrocarbons, which then react with atomic silicon to produce gas-phase SiC2. In our experiments, the silicon carbide nanodust analogues are partially hydrogenated. Chemical routes for the formation of SiC2 and organo-silicon species are discussed on the basis of thermochemical calculations and chemical kinetics modelling. Our findings reveal the central role of molecular hydrogen in the formation of SiC2, and they contribute to a deeper understanding of silicon carbide dust formation processes in evolved stars, from atoms to molecules, clusters and ultimately dust grains.