Elemental nitrogen partitioning in dense interstellar clouds
  Many chemical models of dense interstellar clouds predict that the majority
of gas-phase elemental nitrogen should be present as N2, with an abundance
approximately five orders of magnitude less than that of hydrogen. As a
homonuclear diatomic molecule, N2 is difficult to detect spectroscopically
through infrared or millimetre-wavelength transitions so its abundance is often
inferred indirectly through its reaction product N2H+. Two main formation
mechanisms each involving two radical-radical reactions are the source of N2 in
such environments. Here we report measurements of the low temperature rate
constants for one of these processes, the N + CN reaction down to 56 K. The
effect of the measured rate constants for this reaction and those recently
determined for two other reactions implicated in N2 formation are tested using
a gas-grain model employing a critically evaluated chemical network. We show
that the amount of interstellar nitrogen present as N2 depends on the
competition between its gas-phase formation and the depletion of atomic
nitrogen onto grains. As the reactions controlling N2 formation are
inefficient, we argue that N2 does not represent the main reservoir species for
interstellar nitrogen. Instead, elevated abundances of more labile forms of
nitrogen such as NH3 should be present on interstellar ices, promoting the
eventual formation of nitrogen-bearing organic molecules.
