Revised Models of Interstellar Nitrogen Isotopic Fractionation

TL;DR

This study revises nitrogen isotopic fractionation models in cold molecular clouds, revealing that updated reaction rates fail to reproduce observed $^{15}$N enrichments in molecules like HCN and HNC, indicating the need for alternative mechanisms.

Contribution

The paper introduces updated reaction rates and includes CN fractionation in neutral-neutral reactions, highlighting discrepancies with observations and emphasizing the need for new fractionation processes.

Findings

Revised models suppress $^{15}$N enrichment in ammonia.

No significant $^{15}$N enrichment in HCN or HNC with updated chemistry.

CN can still have low $^{14}$N/$^{15}$N ratios, but models cannot explain observed enrichments.

Abstract

Nitrogen-bearing molecules in cold molecular clouds exhibit a range of isotopic fractionation ratios and these molecules may be the precursors of $^{15}$N enrichments found in comets and meteorites. Chemical model calculations indicate that atom-molecular ion and ion-molecule reactions could account for most of the fractionation patterns observed. However, recent quantum-chemical computations demonstrate that several of the key processes are unlikely to occur in dense clouds. Related model calculations of dense cloud chemistry show that the revised $^{15}$N enrichments fail to match observed values. We have investigated the effects of these reaction rate modifications on the chemical model of Wirstr\"{o}m et al. (2012) for which there are significant physical and chemical differences with respect to other models. We have included $^{15}$N fractionation of CN in neutral-neutral reactions and also updated rate coefficients for key reactions in the nitrogen chemistry. We find that the revised fractionation rates have the effect of suppressing $^{15}$N enrichment in ammonia at all times, while the depletion is even more pronounced, reaching $^{14}$N/$^{15}$N ratios of >2000. Taking the updated nitrogen chemistry into account, no significant enrichment occurs in HCN or HNC, contrary to observational evidence in dark clouds and comets, although the $^{14}$N/$^{15}$N ratio can still be below 100 in CN itself. However, such low CN abundances are predicted that the updated model falls short of explaining the bulk $^{15}$N enhancements observed in primitive materials. It is clear that alternative fractionating reactions are necessary to reproduce observations, so further laboratory and theoretical studies are urgently needed.