Deuterium fractionation of nitrogen hydrides: detections of NHD and ND$_2$

TL;DR

This study detects NHD and ND₂ in the interstellar medium, analyzes their abundance ratios, and compares them with chemical models to understand ammonia formation mechanisms.

Contribution

It provides the first detection of ND₂ in the interstellar medium and compares observed ratios with both gas-phase and grain-surface chemical models.

Findings

NHD and ND₂ detected in absorption towards IRAS16293-2422

Observed abundance ratios are close to 8:4:1 for NH₂:NHD:ND₂

Gas-phase and grain-surface models can reproduce the ratios within a factor of two-three

Abstract

Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD and ND$_2$. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH$_2$ and ND$_2$/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND$_2$ towards the class 0 protostar IRAS16293-2422, towards which NH, NH$_2$ and ND had been previously detected. Both NHD and ND$_2$ are detected in absorption towards the source. The relative abundance ratios NH$_2$ : NHD : ND$_2$ are close to 8 : 4 : 1. These ratios can be reproduced by our gas-phase chemical model within a factor of two-three. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND$_2$ , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry.