2020MNRAS.498.4663L

Carbon containing molecules in cold molecular clouds show various levels of isotopic fractionation through multiple observations. To understand such effects, we have developed a new gas-grain chemical model with updated ¹³C fractionation reactions (also including the corresponding reactions for ¹⁵ N, ¹⁸O, and ³⁴S). For chemical ages typical of dense clouds, our nominal model leads to two ¹³C reservoirs: CO and the species that derive from CO, mainly s-CO and s-CH₃OH, as well as C₃ in the gas phase. The nominal model leads to strong enrichment in C₃, c-C₃H₂, and C₂H in contradiction with observations. When C₃ reacts with oxygen atoms, the global agreement between the various observations and the simulations is rather good showing variable ¹³C fractionation levels that are specific to each species. Alternatively, hydrogen atom reactions lead to notable relative ¹³C 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 ¹³C, particularly CO, depleting atomic ¹³C in the gas phase. This induces a ¹³C 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.