AI 中文总结
研究人员首次在猎户座巴德区探测到星际介质中的C2H+，其作为关键中间体可促进碳氢化合物自下而上增长，相关结果为检验星际分子形成场景提供了重要观测约束。
AI 中文摘要
尽管已在星际介质中探测到近350种分子，其中近一半为碳链，但构建分子复杂性的途径仍知之甚少。碳链和芳香族物种的观测丰度难以与现有自上而下或自下而上的形成场景相契合，部分原因在于观测约束有限及理论认识不完整。特别是小型中间离子是离子-分子反应的关键驱动因素，这类反应能够为更大的碳氢化合物和芳香环提供种子，可为自下而上形成场景提供关键支撑。因此，限制这些离子的丰度和化学性质对于检验自下而上增长是否能在星际条件下有效运行至关重要。在此，我们基于APEX 12米亚毫米望远镜对其最低J=3-2转动跃迁（约211GHz）的观测，报告了在猎户座巴德区（Orion Bar）首次探测到小型碳氢阳离子乙炔鎓（ethynylium，C2H+），该跃迁通过分辨的Λ双重分裂和超精细分裂成分呈现出独特的光谱特征，这些特征已在实验室中得到近期测量。Meudon PDR模型成功重现了这些数值，将C2H+的形成定位于PDR前沿的外边缘。我们的结果将C2H+的产生与振动激发H2驱动的离子-分子反应网络中的CH+和CH3+关联起来，这一情景如今得到了JWST在猎户座巴德等PDR中对这些物种的近期探测的进一步支持。C2H+的重要性在于其作为关键中间体的作用：它生成C2H2+，随后生成C2H3+，有效将小型C2结构单元导向更大的碳氢化合物，并促进PDR表面的自下而上增长。对其他区域中C2H+的针对性搜索有望提供对星际介质中离子驱动的自下而上化学的潜在决定性探测手段。
英文摘要
Despite the detection of nearly 350 molecules in the interstellar medium, almost half of which are carbon chains, the pathways that build molecular complexity remain poorly understood. Observed abundances of carbon-chain and aromatic species are difficult to reconcile with existing top-down or bottom-up formation scenarios, due in part to limited observational constraints and incomplete theoretical understanding. In particular, small intermediary ions, key drivers of ion-molecule reactions capable of seeding larger hydrocarbons and aromatic rings, could provide critical support for the bottom-up formation scenario. Constraining the abundance and chemistry of these ions is therefore essential to test whether bottom-up growth can operate efficiently under interstellar conditions. Here, we report the first detection of the small hydrocarbon cation ethynylium, C2H+, toward the Orion Bar, based on observations with the APEX 12m sub-mm telescope of its lowest-lying J=3-2 rotational transition near 211GHz, which exhibits a unique spectroscopic fingerprint through resolved Lambda-doubling and hyperfine splitting components, as recently measured in the laboratory. Meudon PDR models successfully reproduce these values, placing C2H+ formation at the outer edges of PDR fronts. Our results link C2H+ production to CH+ and CH3+ within a network of ion-molecule reactions driven by vibrationally excited H2, a scenario now further supported by recent detections of these species in PDRs like the Orion Bar with JWST observations. The importance of C2H+ lies in its role as a key intermediate: it produces C2H2+ and subsequently C2H3+, effectively channelling small C2 building blocks toward larger hydrocarbons and facilitating bottom-up growth at the PDR surface. Targeted searches for C2H+ in other regions promise to provide a potentially decisive probe of ion-driven bottom-up chemistry in the ISM.
CommentsAccepted for publication in Astronomy and Astrophysics (19 pages including 6 page Appendix, 13 figures and 3 tables)
