Nitrogen hydrides in the cold envelope of IRAS16293-2422

TL;DR

This study observes nitrogen hydrides in the protostar IRAS16293-2422, providing new measurements of their abundances and revealing discrepancies with existing dark cloud chemical models, especially for NH.

Contribution

First high-resolution observations of nitrogen hydrides in a protostar envelope, offering new data to constrain nitrogen chemistry models.

Findings

NH:NH2:NH3 abundance ratio is 5:1:300.

Observed NH2 and NH3 abundances align with dark cloud models.

NH abundance is significantly underpredicted by models.

Abstract

Nitrogen is the fifth most abundant element in the Universe, yet the gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen hydrides are key molecules of nitrogen chemistry. Their abundance ratios place strong constraints on the production pathways and reaction rates of nitrogen-bearing molecules. We observed the class 0 protostar IRAS16293-2422 with the heterodyne instrument HIFI, covering most of the frequency range from 0.48 to 1.78~THz at high spectral resolution. The hyperfine structure of the amidogen radical o-NH2 is resolved and seen in absorption against the continuum of the protostar. Several transitions of ammonia from 1.2 to 1.8~THz are also seen in absorption. These lines trace the low-density envelope of the protostar. Column densities and abundances are estimated for each hydride. We find that NH:NH2:NH3=5:1:300. {Dark clouds chemical models predict steady-state abundances of NH2 and NH3 in reasonable agreement with the present observations, whilst that of NH is underpredicted by more than one order of magnitude, even using updated kinetic rates. Additional modelling of the nitrogen gas-phase chemistry in dark-cloud conditions is necessary before having recourse to heterogen processes.