在乌云中氮的浓度和H2的正对比例
Nitrogen hydrides and the H2 ortho-to-para ratio in dark clouds
摘要：
含氮物种是各种物体物理条件的常见示踪剂，在乌云中最显着。气态氮的储存器预计是原子或分子，但在暗气体中没有观察到这两种物质。因此，它们的丰度通过化学建模间接来自含氮物种。近年来积累的数据强调了我们对暗云条件下氮化学的不完全理解。为了解决冷气体中氮化学的这个问题，我们已经修改了由关键反应\ce{N+ + H2 -> NH+ + H}引发的氮氧化物的形成。我们提出了一个新的速率，这取决于该反应的邻对位比的H2。这个新的速率允许再现三个氮杂环化合物NH、\ce{NH2}和\ce{NH3}的丰度比，其在IRAS 16293 -2422中观察到，条件是从\ce{N2H+}的解离重组导致NH的通道在低温下不关闭。H2的邻-对位比被限制为O/P= $10^{-3}$的丰度比NH：NH 2，这提供了一种新的方法来测量O/P。这项工作强调需要在低温的乌云的反应速率，和临界解离重组反应的分支比。
英文摘要：
Nitrogen bearing species are common tracers of the physical conditions in a wide variety of objects, and most remarkably in dark clouds. The reservoir of gaseous nitrogen is expected to be atomic or molecular, but none of the two species are observable in the dark gas. Their abundances therefore derive indirectly from those of N-bearing species through chemical modelling. The recent years have accumulated data which stress our incomplete understanding of the nitrogen chemistry in dark cloud conditions. To tackle this problem of the nitrogen chemistry in cold gas, we have revised the formation of nitrogen hydrides, which is initiated by the key reaction \ce{N+ + H2 -> NH+ + H}. We propose a new rate for this reaction which depends on the ortho-to-para ratio of H2. This new rate allows to reproduce the abundance ratios of the three nitrogen hydrides, NH, \ce{NH2}, and \ce{NH3}, observed towards IRAS16293-2422, provided that the channel leading to NH from the dissociative recombination of \ce{N2H+} is not closed at low temperature. The ortho-to-para ratio of H2 is constrained to O/P=$10^{-3}$ by the abundance ratio NH:NH2, which provides a new method to measure O/P. This work stresses the need for reaction rates at the low temperatures of dark clouds, and for branching ratios of critical dissociative recombination reactions.
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