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Prevalence of non-aromatic carbonaceous molecules in the inner regions of circumstellar envelopes

机译：非芳香碳质分子在星际包膜内部区域的盛行

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Evolved stars are foundries of chemical complexity, gas and dust that provide the building blocks of planets and life, and dust nucleation first occurs in their photosphere. The circumstellar regions enveloping these stars, despite their importance, remain hidden to many observations, and dust formation processes are therefore still poorly understood. Laboratory astrophysics provides complementary routes to unveil these chemical processes, but most experiments rely on combustion or plasma decomposition of molecular precursors under physical conditions far removed from those in space. To reproduce and characterize the bottom-up dust formation process, we have built an ultra-high vacuum machine combining atomic gas aggregation with advanced in situ characterization techniques. We show that carbonaceous dust analogues that formed from low-pressure gas-phase condensation of carbon atoms in a hydrogen atmosphere, in a ratio of carbon to molecular hydrogen similar to that reported for evolved stars, lead to the formation of amorphous carbon nanograins and aliphatic carbon clusters. Aromatic species and fullerenes do not form effectively under these conditions, raising implications for a revision of the chemical mechanisms taking place in circumstellar envelopes.Using an atomic gas aggregation process in the laboratory to simulate the conditions in the inner regions of a carbon-rich evolved star, Martinez, Santoro, Merino and colleagues. show that aromatic species and fullerenes form surprisingly inefficiently, and that amorphous carbon nanograins and aliphatic clusters dominate.
机译：演化的恒星是具有化学复杂性，气体和尘埃的铸造厂，它们是构成行星和生命的基础，而尘埃核首先出现在它们的光球中。包围这些恒星的星际区域尽管很重要，但仍然对许多观测隐藏，因此对尘埃形成过程的了解仍然很少。实验室天体物理学提供了揭示这些化学过程的补充途径，但大多数实验都依赖于远离太空的物理条件下分子前体的燃烧或等离子体分解。为了重现和表征自下而上的粉尘形成过程，我们构建了一种超高真空机器，将原子气体聚集与先进的原位表征技术相结合。我们表明，由碳原子在氢气氛中低压气相凝结形成的碳粉尘类似物，其碳与分子氢之比与所报道的恒星相似，导致形成无定形碳纳米粒和脂肪族碳簇。在这些条件下不能有效形成芳香族物种和富勒烯，这对修订星际包膜中的化学机理产生了影响。在实验室中使用原子气体聚集过程模拟富碳演化内部区域的条件明星，马丁内斯，圣托罗，美利奴和同事。结果表明，芳香族物种和富勒烯的形成效率低得令人惊讶，非晶碳纳米颗粒和脂肪族簇占主导地位。

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