2018MNRAS.476.4994F

Partitioning of elemental nitrogen in star-forming regions is not well constrained. Most nitrogen is expected to be partitioned among atomic nitrogen (N I), molecular nitrogen ( N₂), and icy N-bearing molecules, such as NH₃ and N₂. N I is not directly observable in the cold gas. In this paper, we propose an indirect way to constrain the amount of N I in the cold gas of star-forming clouds, via deuteration in ammonia ice, the [ND₂H/NH₂D]/[NH₂D/NH₃] ratio. Using gas-ice astrochemical simulations, we show that if atomic nitrogen remains as the primary reservoir of nitrogen during cold ice formation stages, the [ND₂H/NH₂D]/[NH₂D/NH₃] ratio is close to the statistical value of 1/3 and lower than unity, whereas if atomic nitrogen is largely converted into N-bearing molecules, the ratio should be larger than unity. Observability of ammonia isotopologues in the inner hot regions around low-mass protostars, where ammonia ice has sublimated, is also discussed. We conclude that the [ND₂H/NH₂D]/[NH₂D/NH₃] ratio can be quantified using a combination of Very Large Array and Atacama Large Millimeter/submillimeter Array observations with reasonable integration times, at least towardsIRAS 16293-2422, where high molecular column densities are expected.