Silicon is present in interstellar dust grains, meteorites and asteroids, and to date thirteen silicon-bearing molecules have been detected in the gas-phase towards late-type stars or molecular clouds, including silane and silane derivatives. In this work, we have experimentally studied the interaction between atomic silicon and hydrogen under physical conditions mimicking those at the atmosphere of evolved stars. We have found that the chemistry of Si, H and H$_2$ efficiently produces silane (SiH$_4$), disilane (Si$_2$H$_6$) and amorphous hydrogenated silicon (a-Si:H) grains. Silane has been definitely detected towards the carbon-rich star IRC\,+10216, while disilane has not been detected in space yet. Thus, based on our results, we propose that gas-phase reactions of atomic Si with H and H$_2$ are a plausible source of silane in C-rich AGBs, although its contribution to the total SiH$_4$ abundance may be low in comparison with the suggested formation route by catalytic reactions on the surface of dust grains. In addition, the produced a-Si:H dust analogs decompose into SiH$_4$ and Si$_2$H$_6$ at temperatures above 500 K, suggesting an additional mechanism of formation of these species in envelopes around evolved stars. We have also found that the exposure of these dust analogs to water vapor leads to the incorporation of oxygen into Si-O-Si and Si-OH groups at the expense of SiH moieties, which implies that, if this type of grains are present in the interstellar medium, they will be probably processed into silicates through the interaction with water ices covering the surface of dust grains.

中文翻译：

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模拟演化恒星大气的实验室条件下的硅和氢化学

硅存在于星际尘埃颗粒、陨石和小行星中，迄今为止，已在朝向晚型恒星或分子云的气相中检测到 13 种含硅分子，包括硅烷和硅烷衍生物。在这项工作中，我们在模拟演化恒星大气层的物理条件下，通过实验研究了原子硅和氢之间的相互作用。我们发现 Si、H 和 H$_2$ 的化学作用有效地产生硅烷 (SiH$_4$)、乙硅烷 (Si$_2$H$_6$) 和非晶氢化硅 (a-Si:H) 晶粒。在富含碳的恒星 IRC\,+10216 中已明确检测到硅烷，而在太空中尚未检测到乙硅烷。因此，基于我们的结果，我们提出原子 Si 与 H 和 H$_2$ 的气相反应是富含 C 的 AGB 中硅烷的合理来源，尽管与灰尘颗粒表面催化反应的建议形成路线相比，它对总 SiH$_4$ 丰度的贡献可能较低。此外，产生的 a-Si:H 尘埃类似物在高于 500 K 的温度下分解为 SiH$_4$ 和 Si$_2$H$_6$，表明这些物种在演化恒星周围的包层中形成的额外机制。我们还发现，这些粉尘类似物暴露于水蒸气会导致氧并入 Si-O-Si 和 Si-OH 基团中，但会消耗 SiH 部分，这意味着，如果此类颗粒存在于在星际介质中，它们很可能会通过与覆盖在尘埃颗粒表面的水冰相互作用而被加工成硅酸盐。