Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation
Multiple nitrogen reservoirs in a protoplanetary disk at the epoch of comet and giant planet formation
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DOI：
10.1051/0004-6361/201936750
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发表时间：
2019-12-11
影响因子：
6.5
通讯作者：
Forveille, T.
中科院分区：
文献类型：
--
作者：
Hily-Blant, P.;de Souza, V. Magalhaes;Forveille, T.
在太阳系原始天体中测得的氮同位素比率显示出很大的数值范围，其来源仍然未知。一个关键问题是这些氮同位素库是早于彗星形成阶段还是晚于彗星形成阶段，另一个核心问题是阐明能够产生所观察到的N-14/N-15同位素比值变化的过程。围绕前主序星（金牛座T）运行的卫星为观察原太阳星云类似物的化学成分提供了独特的机会，因此可以建立一个全面的方案，解释太阳系和行星形成盘中氮的起源。有了阿尔马，就有可能测量这种环境中含氮物种的同位素比率。我们目前的光谱和空间分辨观测的超精细结构的4-3旋转跃迁的HCN和它的主要同位素（HCN）-C-13和（HCN）-N-15在磁盘轨道的8百万岁的金牛座T星星TW Hya。灵敏度允许在最小假设的情况下直接测量HCN/（HCN）-C-13和HCN/（HCN）-N-15丰度比。在磁盘上的空间平均，比率分别为86 +/- 4和223 +/- 21。后一个值明显低于该盘中CN/（CN）-N-15的比值323 +/- 30，因此提供了第一个证据，表明在巨行星和彗星形成阶段，盘中存在两个氮同位素库。此外，我们发现明确的证据表明HCN与（HCN）-N-15的比率随半径增加。在外盘，在45 Au，比率为339 +/- 28，在本地星际介质中的直接测量非常一致，并从星系演化计算预测的散装氮同位素比。在r = 20 Au的彗星形成区域，该比率大约低3倍，为121 +/- 11。这种径向增加定性同意的情况下，选择性的光解N-2是占主导地位的分馏过程。然而，我们的同位素比率和HCN发射层的动力学温度定量不同意磁盘中的氮化学模型。
The isotopic ratio of nitrogen measured in primitive Solar System bodies shows a broad range of values, the origin of which remains unknown. One key question is whether these isotopic reservoirs of nitrogen predate the comet formation stage or are posterior to it. Another central question is elucidating the processes that can produce the observed variations in the N-14/N-15 isotopic ratio. Disks that orbit pre-main-sequence (T Tauri) stars provide unique opportunities for observing the chemical content of analogs of the protosolar nebula and therefore for building a comprehensive scenario that can explain the origin of nitrogen in the Solar System and in planet-forming disks. With ALMA, it has become possible to measure isotopic ratios of nitrogen-bearing species in such environments. We present spectrally and spatially resolved observations of the hyperfine structure of the 4-3 rotational transition of HCN and its main isotopologs (HCN)-C-13 and (HCN)-N-15 in the disk orbiting the 8 Myr old T Tauri star TW Hya. The sensitivity allows directly measuring the HCN/(HCN)-C-13 and HCN/(HCN)-N-15 abundance ratios with minimal assumptions. Averaged spatially over the disks, the ratios are 86 +/- 4 and 223 +/- 21, respectively. The latter value is significantly lower than the CN/(CN)-N-15 ratio of 323 +/- 30 in this disk and thus provides the first evidence that two isotopic reservoirs of nitrogen are present in a disk at the stage of giant planet and comet formation. Furthermore, we find clear evidence for an increase in the ratio of HCN to (HCN)-N-15 with radius. The ratio in the outer disk, at 45 au, is 339 +/- 28, in excellent agreement with direct measurements in the local interstellar medium, and with the bulk nitrogen isotopic ratio predicted from galactic evolution calculations. In the comet formation region at r = 20 au, the ratio is a factor approximate to 3 lower, 121 +/- 11. This radial increase qualitatively agrees with the scenario in which selective photodissociation of N-2 is the dominant fractionation process. However, our isotopic ratios and kinetic temperature of the HCN-emitting layers quantitatively disagree with models of nitrogen chemistry in disks.