  Nitrogen-bearing molecules display variable isotopic fractionation levels in
different astronomical environments such as in the interstellar medium or in
the Solar System. Models of interstellar chemistry are unable to induce
nitrogen fraction in cold molecular clouds as exchange reactions for 15N are
mostly inefficient. Here, we developed a new gas-grain model for nitrogen
fractionation including a thorough search for new nitrogen fractionation
reactions and a realistic description of atom depletion onto interstellar dust
particles. We show that, while dense molecular cloud gas-phase chemistry alone
leads to very low fractionation, 14N atoms are preferentially depleted from the
gas-phase due to a mass dependent grain surface sticking rate for atomic
nitrogen. However, assuming an elementary 14N/15N ratio of 441 (equal to the
solar wind value), our model leads to only low 15N enrichment for all
N-containing species synthesized in the gas-phase with predicted 14N/15N ratios
in the range 360-400. Higher enrichment levels can neither be explained by this
mechanism, nor through chemistry, with two possible explanations. (I) The
elementary 14N/15N ratio in the local ISM is smaller, as suggested by the
recent work of Romano et al, with an hypothetic 15NNH+ and 15NNH+ depletion due
to variation of the electronic recombination rate constant variation with the
isotopes. (II) N2 photodissociation leads to variable nitrogen fractionation in
diffuse molecular clouds where photons play an important role, which is
conserved during dense molecular cloud formation as suggested by the work of
Furuya & Aikawa.
