Carbon isotopic fractionation in molecular clouds

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