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arXiv 1208.0192astro-ph.SRastro-ph.EP
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原始太阳系物质的同位素异常：星际云中氮和氘的自旋态分馏

Isotopic Anomalies in Primitive Solar System Matter: Spin-state Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds

E. S. Wirström, S. B. Charnley, M. A. Cordiner, S. N. Milam

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Submitted to ApJL
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摘要：

在陨石和行星际尘埃颗粒中发现的有机物质富含D和15 N。这与携带这些同位素异常的官能团（腈和胺）是由原太阳星云中的离子分子化学形成的想法一致。低温下星际分馏的理论模型预测D和15 N都有很大的富集，可以解释碳质陨石中测得的最大同位素富集。然而，最近的测量表明，在一些原始样品中，大的15 N富集与D中的一个不相关，并且一些富含D的原始材料显示很少（如果有的话）15 N富集。通过考虑自旋状态的依赖性，在涉及的邻位和对位形式的H2的离子-分子反应中，我们表明，氨和相关分子可以表现出这样一个广泛的分馏15 N和D在致密的云核。我们还表明，虽然腈，HCN和HNC，含有最大的15 N富集，这是不相关的极端D富集。因此，这些计算支持太阳系15 N和D同位素异常具有星际遗产的观点。我们还将我们的结果与现有的天文观测结果进行了比较，并简要讨论了未来对该模型的测试。

英文摘要：

Organic material found in meteorites and interplanetary dust particles is enriched in D and 15N. This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar nebula. Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and 15N and can account for the largest isotopic enrichments measured in carbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large 15N enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, 15N enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H2, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both 15N and D in dense cloud cores. We also show that while the nitriles, HCN and HNC, contain the greatest 15N enrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support the view that Solar System 15N and D isotopic anomalies have an interstellar heritage. We also compare our results to existing astronomical observations and briefly discuss future tests of this model.

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