Although ammonia is an abundant molecule commonly observed towards the dense interstellar medium, it has not yet been established whether its main formation route is from gas-phase ion-molecule reactions or grain-surface hydrogen additions on adsorbed nitrogen atoms. Deuterium fractionation can be used as a tool to constrain formation mechanisms. High abundances of deuterated molecules are routinely observed in the dense interstellar medium, with the ratio between deuterated molecules and the main isotopologue enhanced by several orders of magnitude with respect to the elemental D/H ratio. In the case of ammonia, the detection of its triply deuterated isotopologue hints at high abundances of the deuterated intermediate nitrogen radicals, ND, NHD, and ND2. So far however, only ND has been detected in the interstellar medium. In this paper, to constrain the formation of ammonia, we aim at determining the NHD/NH2 and ND2/NHD abundance ratios, and compare them with the predictions of both pure gas-phase and grain-surface chemical models. We searched for the fundamental rotational transitions of NHD and ND2 towards the class 0 protostar IRAS16293-2422, towards which NH, NH2 and ND had been previously detected. Both NHD and ND2 are detected in absorption towards the source. The relative abundance ratios NH2:NHD:ND2 are close to 8:4:1. These ratios can be reproduced by our gas-phase chemical model within a factor of 2-3. Statistical ratios as expected from grain-surface chemistry are also consistent with our data. Further investigations of the ortho-to-para ratio in ND2 , both theoretical and observational, could bring new constraints to better understand nitrogen hydride chemistry.

中文翻译：

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氢化氮的氘分馏：NHD 和 ND2 的检测

尽管氨是在致密星际介质中常见的丰富分子，但尚未确定其主要形成途径是来自气相离子分子反应还是颗粒表面氢吸附在氮原子上。氘分馏可用作限制形成机制的工具。在致密的星际介质中经常观察到高丰度的氘化分子，氘化分子与主要同位素体之间的比率相对于元素 D/H 比增加了几个数量级。在氨的情况下，其三重氘化同位素体的检测暗示了高丰度的氘化中间氮自由基 ND、NHD 和 ND2。然而，到目前为止，仅在星际介质中检测到 ND。在本文中，为了限制氨的形成，我们的目标是确定 NHD/NH2 和 ND2/NHD 丰度比，并将它们与纯气相和颗粒表面化学模型的预测进行比较。我们搜索了 NHD 和 ND2 向 0 类原星 IRAS16293-2422 的基本旋转跃迁，之前已检测到 NH、NH2 和 ND。NHD 和 ND2 都被检测到对源的吸收。NH2:NHD:ND2 的相对丰度比接近 8:4:1。这些比率可以通过我们的气相化学模型在 2-3 倍的范围内重现。晶粒表面化学所预期的统计比率也与我们的数据一致。对 ND2 中邻位与对位比率的进一步研究，包括理论和观察，可以为更好地理解氢化氮化学带来新的限制。