THE mechanisms of interstellar grain formation are not well understood. Such grains are complex1,2, 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 grains9-11. However, the processes leading to the formation of grains are usually discussed only in the general terms of thermodynamics and classical nucleation theory12,13. 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.
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