arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1802.02841astro-ph.SRastro-ph.GA
收藏

原行星盘中的氮同位素分馏

Nitrogen isotope fractionation in protoplanetary disks

Ruud Visser, Simon Bruderer, Paolo Cazzoletti, Stefano Facchini, Alan N. Heays, Ewine F. van Dishoeck

备注：
Accepted by A&A; 17 pages, 17 figures; note added in proof
发表信息：
A&A 615, A75 (2018)
阅读 PDF
arXiv 页面 ↗HTML 原文 ↗

摘要：

目标：氮的两种稳定同位素14 N和15 N在太阳系内外都表现出不同的丰度比。太阳附近的元素比是440。最近的ALMA观测显示，在6个金牛座T和赫比格盘中，HCN/HC 15 N的比率从83到156不等，而在一颗金牛座T恒星中，CN/C15 N的比率为323 +/- 30。我们的目标是确定占主导地位的机制，负责这些增强15 N：低温交换反应或同位素选择性光解N2。方法：使用热化学程序DALI，我们模拟了具有二维轴对称几何形状的星周盘中的氮同位素化学。我们的化学网络是第一个包括氮的分馏机制。该模型产生的几个关键的含氮物种的丰度剖面和同位素比值。我们研究这些同位素比如何依赖于各种磁盘参数。结果：CN和HCN的形成与紫外光照射的表面层中H2的振动激发密切相关。同位素分馏完全由N2的同位素选择性光解控制。在盘的内部100 au的HCN和HC 15 N的柱密度比并不强烈依赖于盘的质量，耀斑角或恒星光谱，但它是敏感的粒度分布。对于较大的颗粒，N2的自屏蔽相对于尘埃消光变得更加重要，导致更强的同位素分馏。在~50和200 Au的盘半径之间，模型预测HCN/HC 15 N和CN/C15 N丰度比与盘和彗星的观测结果一致。模型中的HCN/HC_（15）N和CN/C_（15）N柱密度比比由ALMA观测推断的高2-3倍。

英文摘要：

Aims: The two stable isotopes of nitrogen, 14N and 15N, exhibit a range of abundance ratios both inside and outside the solar system. The elemental ratio in the solar neighborhood is 440. Recent ALMA observations showed HCN/HC15N ratios from 83 to 156 in six T Tauri and Herbig disks and a CN/C15 N ratio of 323 +/- 30 in one T Tauri star. We aim to determine the dominant mechanism responsible for these enhancements of 15N: low-temperature exchange reactions or isotope-selective photodissociation of N2. Methods: Using the thermochemical code DALI, we model the nitrogen isotope chemistry in circumstellar disks with a 2D axisymmetric geometry. Our chemical network is the first to include both fractionation mechanisms for nitrogen. The model produces abundance profiles and isotope ratios for several key N-bearing species. We study how these isotope ratios depend on various disk parameters. Results: The formation of CN and HCN is closely coupled to the vibrational excitation of H2 in the UV-irradiated surface layers of the disk. Isotope fractionation is completely dominated by isotope-selective photodissociation of N2. The column density ratio of HCN over HC15N in the disk's inner 100 au does not depend strongly on the disk mass, the flaring angle or the stellar spectrum, but it is sensitive to the grain size distribution. For larger grains, self-shielding of N2 becomes more important relative to dust extinction, leading to stronger isotope fractionation. Between disk radii of ~50 and 200 au, the models predict HCN/HC15N and CN/C15N abundance ratios consistent with observations of disks and comets. The HCN/HC15N and CN/C15N column density ratios in the models are a factor of 2-3 higher than those inferred from the ALMA observations.

机构、总结、中文摘要均由AI分析而来，可能存在错误，请以论文正文为准。

↑