  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 15N,
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 which 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.
