Radial mixing in protoplanetary accretion disks - III. Carbon dust oxidation and abundance of hydrocarbons in comets

The oxidation of carbon dust and its conversion into CO and some hydrocarbon compounds as intermediate products is studied for protoplanetary accretion discs. The disc model is based on the one-zone alpha-disc approximation. The radial mixing of the combustion products into the cold outer disc region by turbulent diffusion is considered in the transport-diffusion-reaction equations for the chemistry. It is shown that considerable amounts of CH4 and C2H2 formed as by-products of carbon oxidation are mixed into cold disc regions beyond a distance of 10 AU where they can be included into the ice mixture of cometary nuclei formed in that region. This may explain the high abundance of these gases recently observed in comets Hyakutake (C/1996 B2) and Hale-Bopp (C/1995 O1). This mixing of hydrocarbons from carbon oxidation cannot, however, explain the high abundance of some other molecular species like C2H6 or CH3OH, which have to be formed by different processes.