On the chemistry of hydrides of N atoms and O$^+$ ions

TL;DR

This paper explores the formation mechanisms of nitrogen and oxygen hydrides in diffuse interstellar regions, highlighting the significance of dust grain surface reactions for nitrogen hydrides and gas-phase chemistry for oxygen hydrides.

Contribution

It demonstrates that dust grain surface reactions can explain nitrogen hydrides, while oxygen hydrides are accounted for by gas-phase chemistry, clarifying their formation pathways in diffuse clouds.

Findings

Surface reactions on dust grains explain nitrogen hydrides.

Gas-phase chemistry accounts for O$^+$ hydrides.

Hydride formation varies between diffuse and dense clouds.

Abstract

Previous work by various authors has suggested that the detection by Herschel/HIFI of nitrogen hydrides along the low density lines of sight towards G10.6-0.4 (W31C) cannot be accounted for by gas-phase chemical models. In this paper we investigate the role of surface reactions on dust grains in diffuse regions, and we find that formation of the hydrides by surface reactions on dust grains with efficiency comparable to that for H$_2$ formation reconciles models with observations of nitrogen hydrides. However, similar surface reactions do not contribute significantly to the hydrides of O$^+$ ions detected by Herschel/HIFI present along many sight lines in the Galaxy. The O$^+$ hydrides can be accounted for by conventional gas-phase chemistry either in diffuse clouds of very low density with normal cosmic ray fluxes or in somewhat denser diffuse clouds with high cosmic ray fluxes. Hydride chemistry in dense dark clouds appears to be dominated by gas-phase ion-molecule reactions.