Chemical reactions in protoplanetary accretion disks II. Carbon dust oxidation

This paper considers the gas phase chemistry in a protoplanetary accretion disk, especially the chemistry initiated in the gas phase by destruction of dust close to the central star. Slow radial particle transport moves gas and dust from the cold outer parts of a protoplanetary accretion disk into its warm central part where chemical reactions in the gas phase are activated. At the same time gases frozen on the surface of dust grains are vaporized and later the dust grains themselves are vaporized or destroyed by chemical surface reactions. In this paper we take into account oxidation processes of carbon dust by OH molecules and free O atoms. Oxidation by OH molecules turns out to be very efficient and strongly modify the hydrocarbon chemistry in the protoplanetary disk. Due to the slow conversion of hydrocarbons to CO at low temperatures we find that large amounts of methane and more complex organic molecules are formed in the region between the present positions of Venus and Mars. Closer to the protosun, these are converted into CO.