Interstellar chemistry of nitrogen hydrides in dark clouds

Nitrogen, amongst the most abundant metals in the interstellar medium, has a peculiar chemistry that differs from those of carbon and oxygen. Recent observations of several nitrogen-bearing species in the interstellar medium suggest abundances in sharp disagreement with current chemical models. Although some of these observations show that some gas-grain processes are at work, gas-phase chemistry needs first to be revisited. Strong constraints are provided by recent Herschel observations of nitrogen hydrides in cold gas. The aim of the present work is to comprehensively analyse the interstellar chemistry of nitrogen, focussing on the gas-phase formation of the smallest polyatomic species and, in particular, on nitrogen hydrides. We present a new chemical network in which the kinetic rates of critical reactions have been updated based on recent experimental and theoretical studies, including nuclear spin branching ratios. Our network thus treats the different spin symmetries of the nitrogen hydrides self-consistently, together with the ortho and para forms of molecular hydrogen. This new network is used to model the time evolution of the chemical abundances in dark cloud conditions. The steady-state results are analysed, with special emphasis on the influence of the overall amounts of carbon, oxygen, and sulphur. Our calculations are also compared with Herschel/HIFI observations of NH, NH2, and NH3 detected towards the external envelope of the protostar IRAS 16293-2422. The observed abundances and abundance ratios are reproduced for a C/O gas-phase elemental abundance ratio of ~0.8, provided that the sulphur abundance be depleted by a factor greater than 2. The ortho-to-para ratio of H2 in these models is ~ 10-3. Our models also provide predictions for the ortho-to-para ratios of NH2 and NH3 of ~2.3 and ~0.7, respectively. We conclude that the abundances of nitrogen hydrides in dark cloud conditions are consistent with the gas-phase synthesis predicted with our new chemical network