Elemental nitrogen partitioning in dense interstellar clouds

TL;DR

This study investigates the formation and abundance of molecular nitrogen (N2) in dense interstellar clouds, revealing that N2 is not the main nitrogen reservoir and highlighting the importance of other nitrogen compounds like NH3.

Contribution

The paper provides new low-temperature rate constants for the N + CN reaction and assesses their impact on nitrogen chemistry in interstellar clouds.

Findings

N2 formation is inefficient in dense clouds.

Atomic nitrogen depletion onto grains affects N2 abundance.

NH3 and other nitrogen compounds are likely main reservoirs.

Abstract

Many chemical models of dense interstellar clouds predict that the majority of gas-phase elemental nitrogen should be present as N2, with an abundance approximately five orders of magnitude less than that of hydrogen. As a homonuclear diatomic molecule, N2 is difficult to detect spectroscopically through infrared or millimetre-wavelength transitions so its abundance is often inferred indirectly through its reaction product N2H+. Two main formation mechanisms each involving two radical-radical reactions are the source of N2 in such environments. Here we report measurements of the low temperature rate constants for one of these processes, the N + CN reaction down to 56 K. The effect of the measured rate constants for this reaction and those recently determined for two other reactions implicated in N2 formation are tested using a gas-grain model employing a critically evaluated chemical network. We show that the amount of interstellar nitrogen present as N2 depends on the competition between its gas-phase formation and the depletion of atomic nitrogen onto grains. As the reactions controlling N2 formation are inefficient, we argue that N2 does not represent the main reservoir species for interstellar nitrogen. Instead, elevated abundances of more labile forms of nitrogen such as NH3 should be present on interstellar ices, promoting the eventual formation of nitrogen-bearing organic molecules.