Nitrogen hydrides and the H2 ortho-to-para ratio in dark clouds
  Nitrogen bearing species are common tracers of the physical conditions in a
wide variety of objects, and most remarkably in dark clouds. The reservoir of
gaseous nitrogen is expected to be atomic or molecular, but none of the two
species are observable in the dark gas. Their abundances therefore derive
indirectly from those of N-bearing species through chemical modelling. The
recent years have accumulated data which stress our incomplete understanding of
the nitrogen chemistry in dark cloud conditions. To tackle this problem of the
nitrogen chemistry in cold gas, we have revised the formation of nitrogen
hydrides, which is initiated by the key reaction \ce{N+ + H2 -> NH+ + H}. We
propose a new rate for this reaction which depends on the ortho-to-para ratio
of H2. This new rate allows to reproduce the abundance ratios of the three
nitrogen hydrides, NH, \ce{NH2}, and \ce{NH3}, observed towards IRAS16293-2422,
provided that the channel leading to NH from the dissociative recombination of
\ce{N2H+} is not closed at low temperature. The ortho-to-para ratio of H2 is
constrained to O/P=$10^{-3}$ by the abundance ratio NH:NH2, which provides a
new method to measure O/P. This work stresses the need for reaction rates at
the low temperatures of dark clouds, and for branching ratios of critical
dissociative recombination reactions.
