Dust formation in carbon-rich Wolf-Rayet stars I. Chemistry of small carbon clusters and silicon species

The formation of small carbon chains and molecular precursors to silicon ca rbide grains is investigated in the hot, hostile environment of carbon-rich Wolf-Rayet (WC) winds. We consider only WC stars which produce dust on a c ontinuous basis and develop for the first time non-equilibrium, chemical ki netic routes to nucleating dust precursors in the outflow. These can be use d to calculate quantitatively the yield of such dust precursors for various outflow scenarios. Because WC stars have lost all their hydrogen in the WN phase, the chemical processes used in the model involve a pure helium, car bon, oxygen, silicon and sulphur chemistry which resembles that encountered in graphite or metal vaporization experiments in the laboratory. We derive abundances for small linear carbon clusters up to C-6 and silicon-bearing species for various wind parameters and conclude that high-density regions in the form of clumps or discs are of paramount importance to the formation of dust in WC stars.