Revised Models of Interstellar Nitrogen Isotopic Fractionation
  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.
