Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

中文翻译：

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富含 C 的星周包层中的 C、C2 和 C2H2 的宇宙尘埃类似物的化学

星际碳质尘埃主要形成于富含碳的渐近巨星分支（AGB）恒星周围的星周包层的最内部区域。在这些高度化学分层的区域中，原子碳和双原子碳以及乙炔是仅次于 H2 和 CO 的最丰富的物种。在之前的研究中，我们讨论了碳（C 和 C2）与 H2 的化学关系，表明乙炔和脂肪族物种形成在富碳 AGB 的粉尘形成区域中有效，而芳烃则不然。尽管如此，正如之前的实验所示，乙炔仍然是形成线性聚乙炔链、苯和多环芳烃 (PAH) 的关键成分。然而，这些实验没有考虑碳（C 和 C2）与 C2H2 的化学反应。在这项工作中，通过使用足够量的乙炔，我们研究了其与原子和双原子碳的气相相互作用。我们表明，所涉及的化学反应会产生线性聚乙炔链、苯和其他多环芳烃，这些物质在行星状星云的早期演化阶段就以高丰度被观察到。更重要的是，我们发现了不可忽略数量的纯碳簇和氢化碳簇以及具有脂肪族取代的芳族化合物，这两者都是添加原子碳的直接结果。将烷基取代基引入芳族化合物可以通过涉及夺氢随后甲基加成的机制合理化。在气相中检测到的所有物质都被纳入纳米尺寸的灰尘类似物中，该类似物由具有无定形形态的 sp、sp2 和 sp3 碳氢化合物的复杂混合物组成。