Formation of Complex Organic Molecules in Cold Interstellar Environments through Nondiffusive Grain-surface and Ice-mantle Chemistry
Formation of Complex Organic Molecules in Cold Interstellar Environments through Nondiffusive Grain-surface and Ice-mantle Chemistry
复制标题
DOI：
10.3847/1538-4365/ab9ec8
复制
发表时间：
2020-08-01
影响因子：
8.7
通讯作者：
Garrod, Robin T.
中科院分区：
文献类型：
--
作者：
Lin, Mihwa;Garrod, Robin T.
星际间复杂有机分子（COMs）的普遍理论涉及在温暖的尘埃颗粒表面上形成，通过颗粒表面光解稳定分子产生的自由基的扩散和反应。然而，一些气相含氧化合物，特别是乙醛（CH 3CHO），甲酸甲酯（CH 3 OCHO）和二甲醚（CH 3 OCH 3），现在在非常低的温度下观察到，挑战温暖的情况。在这里，我们介绍了一个新的非扩散机制的选择到天体化学模型来解释失败的标准扩散图片，并提供一个更普遍的情况下COM形成星际颗粒。提供了新的通用速率公式的情况下，其中（i）自由基形成的反应发生接近另一个反应物，产生立即后续反应;（ii）自由基形成的激发态，使它们能够克服活化障碍与附近的稳定分子反应;和（iii）自由基形成通过光解离接近反应伴侣，然后立即反应。每个过程都在没有大自由基热扩散的情况下发生。新的机制显着提高冷COM丰度，成功地再现了星前核心L1544的关键观测结果。H的提取从粮食表面COM，其次是重组，在放大化学解吸到气相中起着至关重要的作用。紫外线诱导的化学物质产生显着COM丰度散装冰，这是保留在谷物，并可能持续到后期阶段。O（2）也是通过光分解在地幔中强烈形成的，这表明彗星O（2）确实可能是星际的。
A prevailing theory for the interstellar production of complex organic molecules (COMs) involves formation on warm dust-grain surfaces, via the diffusion and reaction of radicals produced through grain-surface photodissociation of stable molecules. However, some gas-phase O-bearing COMs, notably acetaldehyde (CH3CHO), methyl formate (CH3OCHO), and dimethyl ether (CH3OCH3), are now observed at very low temperatures, challenging the warm scenario. Here, we introduce a selection of new nondiffusive mechanisms into an astrochemical model to account for the failure of the standard diffusive picture and to provide a more generalized scenario of COM formation on interstellar grains. New generic rate formulations are provided for cases where (i) radicals are formed by reactions occurring close to another reactant, producing an immediate follow-on reaction; (ii) radicals are formed in an excited state, allowing them to overcome activation barriers to react with nearby stable molecules; and (iii) radicals are formed through photodissociation close to a reaction partner, followed by immediate reaction. Each process occurs without thermal diffusion of large radicals. The new mechanisms significantly enhance cold COM abundances, successfully reproducing key observational results for prestellar coreL1544. H abstraction from grain-surface COMs, followed by recombination, plays a crucial role in amplifying chemical desorption into the gas phase. The ultraviolet-induced chemistry produces significant COM abundances in the bulk ices, which are retained on the grains and may persist to later stages. O(2)is also formed strongly in the mantle though photolysis, suggesting cometary O(2)could indeed be interstellar.