Silicon carbide and the origin of interstellar carbon grains

THE mechanisms of interstellar grain formation are not well understood. Such grains are complex^1,2, with components that include silicates, ices and carbonaceous material. Several forms have been proposed for interstellar carbon, including graphite³, amorphous carbon⁴, polycyclic aromatic hydrocarbons (PAHs)⁵, diamond⁶, fullerenes⁷ and bacteria⁸. The stellar winds of carbon-rich red-giant stars are the dominant source of interstellar carbon grains^9-11. However, the processes leading to the formation of grains are usually discussed only in the general terms of thermodynamics and classical nucleation theory^12,13. One possibility⁹ is that PAH condensation initiates the formation of grains, similar to the formation of soot in hydrocarbon pyrolysis and combustion. Our recent computational study¹⁴ demonstrates by detailed chemical kinetics modelling that PAHs can, under certain conditions, form in carbon-star molecular envelopes within a 900-1,100-K temperature regime. Although homogeneous gas-phase condensation of PAHs might lead to the formation of grains, we suggest instead that SiC nucleates at higher temperatures and provides a surface for subsequent carbon condensation. Here we present experimental evidence in support of this idea.