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

TL;DR

This study proposes an indirect method using ammonia deuteration ratios to estimate atomic nitrogen abundance in star-forming regions, which is otherwise difficult to observe directly in cold gas.

Contribution

It introduces a novel astrochemical simulation approach linking ammonia deuteration ratios to atomic nitrogen levels in star-forming clouds.

Findings

Deuteration ratio close to 1/3 indicates atomic nitrogen as the main reservoir.

Ratios greater than unity suggest conversion of atomic nitrogen into N-bearing molecules.

Method feasible with VLA and ALMA observations, especially toward IRAS 16293-2422.

Abstract

Partitioning of elemental nitrogen in star-forming regions is not well constrained. Most nitrogen is expected to be partitioned among atomic nitrogen, molecular nitrogen (N2), and icy N-bearing molecules, such as ammonia (NH3) and N2. Atomic nitrogen is not directly observable in the cold gas. In this paper, we propose an indirect way to constrain the amount of atomic nitrogen 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 VLA and ALMA observations with reasonable integration times, at least toward IRAS 16293-2422 where high molecular column densities are expected.