Tracing the atomic nitrogen abundance in star-forming regions with ammonia deuteration

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 (N 2 ), and icy N-bearing molecules, such as NH 3 and N 2 . NI is not directly observable in the cold gas. In this paper, we propose an indirect way to constrain the amount of NI in the cold gas of star-forming clouds, via deuteration in ammonia ice, the [ND 2 H/NH 2 D]/[NH 2 D/NH 3 ] 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 [ND 2 H/NH 2 D]/[NH 2 D/NH 3 ] ratio is close to the statistical value of 1/3 and lower than unity, whereas if atomic nitrogen is largely converted into N-bearingmolecules, 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 [ND 2 H/NH 2 D]/[NH 2 D/NH 3 ] ratio can be quantified using a combination of Very Large Array and Atacama Large Millimeter/ submillimeter Array observations with reasonable integration times, at least towards IRAS 16293-2422, where high molecular column densities are expected.