Nitrogen hydrides and the H-2 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 that 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 N+ + H-2 -> NH+ + H. We propose a new rate for this reaction that depends on the ortho-to-para ratio of H-2. This new rate allows reproduction of the abundance ratios of the three nitrogen hydrides, NH, NH2, and NH3, which are observed towards IRAS 16293-2422, provided that the channel leading to NH from the dissociative recombination of N2H+ is not closed at low temperature. The ortho-to-para ratio of H-2 is constrained to O/P = 10(-3) by the abundance ratio NH:NH2, which provides a new method of measuring 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.