Nitrogen hydrides and the H₂ ortho-to-para ratio in dark clouds

¹ Université Joseph Fourier and CNRS, Institut de Planétologie et d’Astrophysique, Grenoble, France
e-mail: pierre.hily-blant@obs.ujf-grenoble.fr
² Université Paris-Orsay and CNRS, Institut d’Astrophysique Spatiale, Orsay, France

Received: 25 July 2011
Accepted: 19 October 2011

Abstract

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⁺ + H₂ → NH⁺ + H. We propose a new rate for this reaction that depends on the ortho-to-para ratio of H₂. 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 H₂ 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.