Carbon isotopic fractionation in molecular clouds

Context. Carbon fractionation has been studied from a theoretical point of view with different models of time-dependent chemistry, including both isotope-selective photodissociation and low-temperature isotopic exchange reactions.Aims. Recent chemical models predict that isotopic exchange reactions may lead to a depletion of C-13 in nitrile-bearing species, with C-12/C-13 ratios two times higher than the elemental abundance ratio of 68 in the local interstellar medium. Since the carbon isotopic ratio is commonly used to evaluate the N-14/N-15 ratios with the double-isotope method, it is important to study carbon fractionation in detail to avoid incorrect assumptions.Methods. In this work, we implemented a gas-grain chemical model with new isotopic exchange reactions and investigated their introduction in the context of dense and cold molecular gas. In particular, we investigated the C-12/C-13 ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2 x 10(3) to 2 x 10(7) cm(-3), respectively.Results. We suggest a possible C-13 exchange through the C-13 + C-3 -> C-12 +(CC2)-C-13 reaction, which does not result in dilution, but rather in C-13 enhancement, for molecules that are formed starting from atomic carbon. This effect is efficient in a range of time between the formation of CO and its freeze-out on grains. Furthermore, the parameter-space exploration shows, on average, that the C-12/C-13 ratios of nitriles are predicted to be a factor 0.8-1.9 different from the local C-12/C-13 of 68 for high-mass star-forming regions. This result also affects the N-14/N-15 ratio: a value of 330 obtained with the double-isotope method is predicted to vary in the range 260-630, up to 1150, depending on the physical conditions. Finally, we studied the C-12/C-13 ratios of nitriles by varying the cosmic-ray ionisation rate, zeta: the C-12/C-13 ratios increase with zeta because of secondary photons and cosmic-ray reactions.