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

TL;DR

This study demonstrates that molecular hydrogen plays a crucial role in forming silicon carbide dust in evolved stars, with SiC₂ identified as a key precursor in laboratory simulations.

Contribution

The paper provides experimental and theoretical evidence for hydrogen's role in silicon carbide dust formation, highlighting SiC₂ as a molecular precursor.

Findings

SiC₂ identified as a key molecular precursor of silicon carbide dust.

Hydrogen interaction initiates hydrocarbon formation leading to SiC₂ production.

Silicon carbide nanodust analogues are partially hydrogenated in experiments.

Abstract

Cosmic dust is mainly formed 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 still 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 H$_2$ and we have clearly identified SiC$_2$ 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 SiC$_2$. In our experiments, the silicon carbide nanodust analogues are partially hydrogenated. Chemical routes for the formation of SiC$_2$ and organosilicon 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 SiC$_2$ and contribute to a deeper understanding of silicon carbide dust formation processes in evolved stars, from atoms to molecules, clusters, and ultimately dust grains.