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arXiv 1606.01049astro-ph.GA
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混合CO、H2CO和CH3OH冰中氢原子的加成和夺取反应:复杂有机分子形成的扩展观点

H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices: an extended view on complex organic molecule formation

K. -J. Chuang, G. Fedoseev, S. Ioppolo, E. F. van Dishoeck, H. Linnartz

摘要:

复杂有机分子不仅存在于低质量和高质量原恒星周围的热核中,而且存在于冷的暗云中。因此,了解如何在不存在嵌入式能源的情况下形成这种物质是很有趣的。我们提出了新的实验室实验,研究了三种复杂分子的低温固态形成:甲酸甲酯(HC(O)OCH 3)、乙醇醛(HC(O)CH 2 OH)和乙二醇(H2 C(OH)CH 2 OH),通过自由基的重组形成。15 K下CO-H2 CO-CH 3OH加氢网络中不同阶段的H原子加成和夺取反应。实验扩展了以前的CO氢化研究,旨在模拟暗分子云中CO冻结阶段的典型物理和化学条件,此时H2 CO和CH 3OH通过在冰粒上的吸积CO分子和H原子的重组形成。我们证实,H2 CO,一旦通过CO加氢形成,不仅产生CH 3OH通过正在进行的H原子加成反应,但也受到H原子诱导的抽象反应,再次产生CO。以类似的方式,H2 CO也在涉及CH 3OH的提取反应中形成。预计主要的甲醇H-原子提取产物是CH 2 OH,而H-原子加成到H2 CO中应至少部分通过CH 3 O中间自由基进行。在冰幔中发生的氢原子提取反应导致比以前认为的更多的反应性中间体(HCO,CH 3 O和CH 2 OH),当假设连续的氢原子加成反应时。这提高了通过自由基-自由基复合形成COM的可能性,而不需要UV光解或宇宙射线作为外部触发。

英文摘要:

Complex organic molecules (COMs) have been observed not only in the hot cores surrounding low- and high- mass protostars, but also in cold dark clouds. Therefore, it is interesting to understand how such species can be formed without the presence of embedded energy sources. We present new laboratory experiments on the low-temperature solid state formation of three complex molecules: methyl formate (HC(O)OCH3), glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH), through recombination of free radicals formed via H-atom addition and abstraction reactions at different stages in the CO-H2CO-CH3OH hydrogenation network at 15 K. The experiments extend previous CO hydrogenation studies and aim at resembling the physical&chemical conditions typical of the CO freeze-out stage in dark molecular clouds, when H2CO and CH3OH form by recombination of accreting CO molecules and H-atoms on ice grains. We confirm that H2CO, once formed through CO hydrogenation, not only yields CH3OH through ongoing H-atom addition reactions, but is also subject to H-atom-induced abstraction reactions, yielding CO again. In a similar way, H2CO is also formed in abstraction reactions involving CH3OH. The dominant methanol H-atom abstraction product is expected to be CH2OH, while H-atom additions to H2CO should at least partially proceed through CH3O intermediate radicals. The occurrence of H-atom abstraction reactions in ice mantles leads to more reactive intermediates (HCO, CH3O and CH2OH) than previously thought, when assuming sequential H-atom addition reactions only. This enhances the probability to form COMs through radical-radical recombination without the need of UV photolysis or cosmic rays as external triggers.

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