用氨氘代追踪恒星形成区的氮原子丰度
Tracing the atomic nitrogen abundance in star-forming regions with ammonia deuteration
摘要：
在恒星形成区的元素氮的分配没有得到很好的约束。预计大多数氮被分配在原子氮、分子氮（N2）和冰冷的含氮分子（例如氨（NH3）和N2）之间。在冷气体中不能直接观察到氮原子。在本文中，我们提出了一种间接的方式来限制恒星形成云的冷气中的原子氮的量，通过在氨冰中的氘化，[ND 2 H/NH 2D]/[NH 2D/NH 3]比。利用气-冰天体化学模拟，我们表明，如果原子氮仍然作为氮的主要水库在冷冰形成阶段，[ND 2 H/NH 2D]/[NH 2D/NH 3]的比例是接近1/3的统计值，低于1，而如果原子氮主要转化为含氮分子，该比例应该大于1。氨同位素在低质量原恒星周围的内部热区域，其中氨冰升华，可观测性进行了讨论。我们的结论是，[ND 2 H/NH 2D]/[NH 2D/NH3]比值可以使用VLA和ALMA观察的组合进行定量，具有合理的积分时间，至少对IRAS 16293-2422，其中预期高分子柱密度。
英文摘要：
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.
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