星际分子云中甲烷表面形成的实验研究
An experimental study of the surface formation of methane in interstellar molecular clouds
摘要：
甲烷是最简单的稳定分子之一，在空间中既丰富又广泛分布。它被认为部分起源于星际分子云，这些分子云接近恒星形成周期的开始。对低质量和高质量年轻恒星天体的CH $_4$冰的观测调查表明，CH $_4$的大部分预计是由尘埃颗粒上的C氢化形成的，CH $_4$冰与固体H $_2$ O密切相关。然而，这还没有在受控的实验室条件下进行研究，因为星际冰类似物的碳原子化学还没有在实验中实现。在这项研究中，我们成功地证明了在两个单独的共沉积实验中，在真空装置中实施的C原子束形成CH $_4$冰：C + H在10 K表面上模拟CH $_4$的形成，正好在H $_2$ O冰在尘埃颗粒上形成之前，和C + H + H $_2$ O在10 K表面上的反应，以模拟与H $_2$ O冰同时形成的CH $_4$。我们证实，CH $_4$可以通过原子C和H的反应形成，并且当CH $_4$在富含H $_2$ O的冰中形成时，CH $_4$的形成速率是2倍。这与星际CH $_4$和H $_2$ O在极地冰相中一起形成的观测发现一致，即，当碳原子和氢原子同时与尘埃颗粒上的氧原子共生时。第一次，导致星际CH $_4$（和CD $_4$）冰形成的条件被报道，并可以纳入天体化学模型，以进一步限制CH $_4$化学在星际介质和CH $_4$继承的其他地区。
英文摘要：
Methane is one of the simplest stable molecules that is both abundant and widely distributed across space. It is thought to have partial origin from interstellar molecular clouds, which are near the beginning of the star formation cycle. Observational surveys of CH$_4$ ice towards low- and high-mass young stellar objects showed that much of the CH$_4$ is expected to be formed by the hydrogenation of C on dust grains, and that CH$_4$ ice is strongly correlated with solid H$_2$O. Yet, this has not been investigated under controlled laboratory conditions, as carbon-atom chemistry of interstellar ice analogues has not been experimentally realized. In this study, we successfully demonstrate with a C-atom beam implemented in an ultrahigh vacuum apparatus the formation of CH$_4$ ice in two separate co-deposition experiments: C + H on a 10 K surface to mimic CH$_4$ formation right before H$_2$O ice is formed on the dust grain, and C + H + H$_2$O on a 10 K surface to mimic CH$_4$ formed simultaneously with H$_2$O ice. We confirm that CH$_4$ can be formed by the reaction of atomic C and H, and that the CH$_4$ formation rate is 2 times greater when CH$_4$ is formed within a H$_2$O-rich ice. This is in agreement with the observational finding that interstellar CH$_4$ and H$_2$O form together in the polar ice phase, i.e., when C- and H-atoms simultaneously accrete with O-atoms on dust grains. For the first time, the conditions that lead to interstellar CH$_4$ (and CD$_4$) ice formation are reported, and can be incorporated into astrochemical models to further constrain CH$_4$ chemistry in the interstellar medium and in other regions where CH$_4$ is inherited.
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