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

TL;DR

This paper revises nitrogen hydride formation models in dark clouds by introducing a new reaction rate dependent on H2's ortho-to-para ratio, enabling better predictions of observed abundances and offering a new method to measure this ratio.

Contribution

It presents a new reaction rate for nitrogen hydride formation that depends on H2's ortho-to-para ratio, improving chemical modeling accuracy in dark clouds.

Findings

The new reaction rate reproduces observed nitrogen hydride ratios.

The ortho-to-para ratio of H2 is constrained to 10^-3.

Highlights the importance of low-temperature reaction rates and branching ratios.

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 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.