Combined model for 15N, 13C, and spin-state chemistry in molecular clouds

We present a new gas-grain chemical model for the combined isotopic fractionation of carbon and nitrogen in molecular clouds. To this end, we have developed gas-phase and grain-surface chemical networks where the isotope chemistry of carbon and nitrogen is coupled with a time-dependent description of spin-state chemistry, which is important for nitrogen chemistry at low temperatures. We updated the rate coefficients of some isotopic exchange reactions considered previously in the literature, and here we present a set of new exchange reactions involving molecules substituted in C-13 and N-15 simultaneously. We applied the model to a series of zero-dimensional simulations representing a set of physical conditions across a prototypical prestellar core, exploring the deviations of the isotopic abundance ratios in the various molecules from the elemental isotopic ratios as a function of physical conditions and time. We find that the C-12/C-13 ratio can deviate from the elemental ratio to a high factor depending on the molecule, and that there are highly time-dependent variations in the ratios. The HCN/(HCN)-C-13 ratio, for example, can obtain values of less than ten depending on the simulation time. The N-14/N-15 ratios tend to remain close to the assumed elemental ratio within approximately 10%, with no clearly discernible trends for the various species as a function of the physical conditions. Abundance ratios between C-13-containing molecules and C-13+15N-containing molecules however show somewhat increased levels of fractionation as a result of the newly included exchange reactions, though they still remain within a few tens of percent of the elemental N-14/N-15 ratio. Our results imply the existence of gradients in isotopic abundance ratios across prestellar cores, suggesting that detailed simulations are required to interpret observations of isotopically substituted molecules correctly, especially given that the various isotopic forms of a given molecule do not necessarily trace the same gas layers.