论文标题
高度振动激发的H2引发N化学的作用：量子研究和3-岩石发射在Orion Bar PDR中的作用
The role of highly vibrationally excited H2 initiating the N chemistry: Quantum study and 3-sigma detection of NH emission in the Orion Bar PDR
论文作者
论文摘要
H2和重元素原子之间的气相反应形成氢化物是一个非常选择性的过程。与地面中性碳，氧，氮和硫原子的反应非常具有内部能力，并且具有高能屏障，因为在形成氢化物键之前必须将H2分子碎裂。在冷星云中，这些障碍不包括通过氢抽象反应的CH，OH，NH和SH自由基的形成。在这里，我们研究了一个非常不利的过程，即n（4s）原子与H2分子的反应。我们使用V = 0-7的量子方法和准经典方法在不同的振动水平下计算了H2的反应速率系数，最高为V = 12。由于高能屏障，这些速率系数随V以及气温而增加。我们在Meudon PDR代码中实施了新的价格，并研究了它们对具有不同紫外线（UV）照明条件的模型的影响。在强烈的紫外线辐射密度（猎户座条的条件）中，与使用反应n（4s） + H2-> nh + H的热速率系数相比，高度振动激发水平（V> 7）中H2的存在增强了NH的丰度（在PDR表面），增强了NH的丰度（在PDR表面）。我们探讨了Orion Bar和Horsehead PDR的现有Herschel/Hifi观察。我们在NH n_j的〜974 GHz频率上报告了一个3- sigma排放功能，朝向栏。发射水平意味着n（nh）〜10^13 cm^-2，它与使用新速率系数的PDR模型一致，用于N和UV泵式H2之间的反应。这种地层途径占主导地位，涉及涉及较少n+离子的氢化反应。 JWST的观察很快将量化许多星际和情节环境中紫外线泵的H2的数量和反应性。
The formation of hydrides by gas-phase reactions between H2 and a heavy element atom is a very selective process. Reactions with ground-state neutral carbon, oxygen, nitrogen, and sulfur atoms are very endoergic and have high energy barriers because the H2 molecule has to be fragmented before a hydride bond is formed. In cold interstellar clouds, these barriers exclude the formation of CH, OH, NH, and SH radicals through hydrogen abstraction reactions. Here we study a very energetically unfavorable process, the reaction of N(4S) atoms with H2 molecules. We calculated the reaction rate coefficient for H2 in different vibrational levels, using quantum methods for v=0-7 and quasi-classical methods up to v=12. Owing to the high energy barrier, these rate coefficients increase with v and also with the gas temperature. We implemented the new rates in the Meudon PDR code and studied their effect on models with different ultraviolet (UV) illumination conditions. In strongly UV-irradiated dense gas (Orion Bar conditions), the presence of H2 in highly vibrationally excited levels (v>7) enhances the NH abundance by two orders of magnitude (at the PDR surface) compared to models that use the thermal rate coefficient for reaction N(4S) + H2 -> NH + H. The increase in NH column density across the PDR is a factor of ~25. We explore existing Herschel/HIFI observations of the Orion Bar and Horsehead PDRs. We report a 3-sigma emission feature at the ~974 GHz frequency of the NH N_J=1_2-0_1 line toward the Bar. The emission level implies N(NH)~10^13 cm^-2, which is consistent with PDR models using the new rate coefficients for reactions between N and UV-pumped H2. This formation route dominates over hydrogenation reactions involving the less abundant N+ ion. JWST observations will soon quantify the amount and reactivity of UV-pumped H2 in many interstellar and circumstellar environments.
