Formation of COMs through CO hydrogenation on interstellar grains
Formation of COMs through CO hydrogenation on interstellar grains
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DOI：
10.1051/0004-6361/201936522
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发表时间：
2020-01
影响因子：
6.5
通讯作者：
M. Simons;T. Lamberts;H. Cuppen
中科院分区：
文献类型：
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作者：
M. Simons;T. Lamberts;H. Cuppen
上下文乙醛、乙二醇和甲酸甲酯是在暗分子云中观察到的复杂有机分子。由于没有有效的气相路线来产生这些物质，因此预计存在不需要高能处理的低温表面路线。低温CO加氢实验表明，情况确实如此。乙二醛可以通过两个HCO基团的重组形成，然后进一步氢化。目标。在这里，我们的目标是通过这种寒冷和黑暗的形成路线来限制星际尘埃颗粒表面上的甲酸甲酯，乙醇醛和乙二醇的形成。我们还探讨了初始气相组成和尘埃温度上的晶粒地幔成分的依赖性。方法.基于速率常数和分支比的量子化学计算，建立了一个完整的CO加氢反应网络。该网络与微观动力学蒙特卡罗模拟结合使用，以模拟冰化学，考虑到所有的位置信息。在对CO加氢实验的模型进行基准测试后，进行了分子云条件下的模拟。结果在我们研究的所有星际条件下，甚至在低至8 K的温度下，都会形成乙醇醛、乙二醇和甲酸甲酯。这是因为当HCO自由基彼此靠近形成并且不需要扩散时，可以发生HCO + HCO反应。形成相对低丰度的甲酸甲酯。最终COM丰度更多地取决于H-CO比，而较少地取决于温度。只有在16 K以上，CO的积累效率较低，温度才开始发挥作用。分子氢主要通过表面上的夺取反应形成。生成甲醇的最重要的反应是H2 CO + CH 3 O → HCO + CH 3OH。我们的模拟与观察到的COM比率已在低温下形成的地幔。
Context. Glycoaldehyde, ethylene glycol, and methyl formate are complex organic molecules that have been observed in dark molecular clouds. Because there is no efficient gas-phase route to produce these species, it is expected that a low-temperature surface route existst that does not require energetic processing. CO hydrogenation experiments at low temperatures showed that this is indeed the case. Glyoxal can form through recombination of two HCO radicals and is then further hydrogenated. Aims. Here we aim to constrain the methyl formate, glycolaldehyde, and ethylene glycol formation on the surface of interstellar dust grains through this cold and dark formation route. We also probe the dependence of the grain mantle composition on the initial gas-phase composition and the dust temperature. Methods. A full CO hydrogenation reaction network was built based on quantum chemical calculations for the rate constants and branching ratios. This network was used in combination with a microscopic kinetic Monte Carlo simulation to simulate ice chemistry, taking into account all positional information. After benchmarking the model against CO-hydrogenation experiments, simulations under molecular cloud conditions were performed. Results. Glycoaldehyde, ethylene glycol, and methyl formate are formed in all interstellar conditions we studied, even at temperatures as low as 8 K. This is because the HCO + HCO reaction can occur when HCO radicals are formed close to each other and do not require to diffuse. Relatively low abundances of methyl formate are formed. The final COM abundances depend more on the H-to-CO ratio and less on temperature. Only above 16 K, where CO build-up is less efficient, does temperature start to play a role. Molecular hydrogen is predominantly formed through abstraction reactions on the surface. The most important reaction leading to methanol is H2CO + CH3O → HCO + CH3OH. Our simulations are in agreement with observed COM ratios for mantles that have been formed at low temperatures.