Gas-grain model of carbon fractionation in dense molecular clouds

Carbon containing molecules in coldmolecular clouds showvarious levels of isotopic fractionation through multiple observations. To understand such effects, we have developed a new gas-grain chemical model with updated C-13 fractionation reactions (also including the corresponding reactions for (15) N, O-18, and S-34). For chemical ages typical of dense clouds, our nominal model leads to two C-13 reservoirs: CO and the species that derive from CO, mainly s-CO and s-CH3OH, as well as C-3 in the gas phase. The nominal model leads to strong enrichment in C-3, c-C3H2, and C2H in contradiction with observations. When C-3 reacts with oxygen atoms, the global agreement between the various observations and the simulations is rather good showing variable C-13 fractionation levels that are specific to each species. Alternatively, hydrogen atom reactions lead to notable relative C-13 fractionation effects for the two non-equivalent isotopologues of C2H, c-C3H2, and C2S. As there are several important fractionation reactions, some carbon bearing species are enriched in C-13, particularly CO, depleting atomic C-13 in the gas phase. This induces a C-13 depletion in CH4 formed on grain surfaces, an effect that is not observed in the CH4 in the Solar system, in particular on Titan. This seems to indicate a transformation of matter between the collapse of the molecular clouds, leading to the formation of the protostellar disc, and the formation of the planets. Or it means that the atomic carbon sticking to the grains reacts with the species already on the grains giving very little CH4.