在CRL 618中搜索邻苯炔（o-C6H4）
A search for ortho-benzyne (o-C6H4) in CRL 618
摘要：
多环芳烃（PAHs）被认为是未识别红外波段（UIR）和星际漫谱带（DIB）的潜在载体。多环芳烃不太可能通过气相或固态星际化学形成，而是可能在富含碳的演化恒星的流出物中产生。多环芳烃可以通过乙炔与苯基（C6 H5）加成形成，苯基与苯（C6 H6）和邻苯炔（o-C6 H4）在化学上密切相关。迄今为止，环星化学模型仅限于苯相关化学的部分处理，因此这些物种的预期丰度尚不清楚。据报道，在原行星状星云（PPN）CRL 618的包层中检测到了苯，但在该来源或任何其他来源中尚未检测到其他与苯相关的物质。o-C6 H4的光谱比C6 H5的光谱明显更简单和更强，因此我们用绿岸望远镜对o-C6 H4进行了深Ku，K和Q波段的搜索。没有检测到跃迁，但确定了柱密度的上限为8.4 × 10^13 cm^-2。这个限制可以用来限制化学模型的PPNe，这项研究表明，需要完全修订这些模型，包括全套苯相关的化学。
英文摘要：
Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar bands (DIBs). PAHs are not likely to form by gas-phase or solid-state interstellar chemistry, but rather might be produced in the outflows of carbon-rich evolved stars. PAHs could form from acetylene addition to the phenyl radical (C6H5), which is closely chemically related to benzene (C6H6) and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been limited to only a partial treatment of benzene-related chemistry, and so the expected abundances of these species are unclear. A detection of benzene has been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but no other benzene-related species has been detected in this or any other source. The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5, and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green Bank Telescope. No transitions were detected, but an upper limit on the column density of 8.4x10^13 cm^-2 has been determined. This limit can be used to constrain chemical models of PPNe, and this study illustrates the need for complete revision of these models to include the full set of benzene-related chemistry.
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