Tracing the atomic nitrogen abundance in star-forming regions with ammonia deuteration
Furuya, Kenji, & Persson, Magnus V.
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. 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 [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-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 [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.