Carbon isotopic fractionation in molecular clouds

Abstract

C-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. Recent chemical models predict that the latter may lead to a depletion of ${}^{13}$C in nitrile-bearing species, with ${}^{12}$C/${}^{13}$C ratios two times higher than the elemental abundance ratio of 68 in the local ISM. Since the carbon isotopic ratio is commonly used to evaluate the ${}^{14}$N/${}^{15}$N ratios with the double-isotope method, it is important to study C-fractionation in detail to avoid incorrect assumptions. 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 ${}^{12}$C/${}^{13}$C ratios of HNC, HCN, and CN using a grid of models, with temperatures and densities ranging from 10 to 50 K and 2$\times$10${}^3$ to 2$\times$10${}^7$ cm${}^{-3}$, respectively. We suggest a possible ${}^{13}$C exchange through the ${}^{13}$C + C${}_3$ $\to$ ${}^{12}$C +${}^{13}$CC${}_2$ reaction, which does not result in dilution, but rather in ${}^{13}$C enhancement, for molecules 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, we show that the ${}^{12}$C/${}^{13}$C ratios of nitriles are predicted to be a factor 0.8-1.9 different from the local value of 68 for massive star-forming regions. This result also affects the ${}^{14}$N/${}^{15}$N ratio: a value of 330 obtained with the double-isotope method is predicted to be 260-1150, depending on the physical conditions. Finally, we studied the ${}^{12}$C/${}^{13}$C ratios by varying the cosmic-ray ionization rate: the ratios increase with it because of secondary photons and cosmic-ray reactions.

arXiv commentThe published exponent in the A&A journal of the second rate coefficient of Table 1 is a misprint

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