PDRs4All. XII. 远紫外驱动的烃自由基形成及其与多环芳烃的关系
PDRs4All. XII. FUV-driven formation of hydrocarbon radicals and their relation with PAHs
AI总结：
本文利用ALMA和JWST观测猎户座棒PDR，揭示远紫外驱动的烃自由基形成与PAHs脂肪族侧基之间的化学关联，为分子云中碳载体演化提供空间分辨证据。
摘要：
我们展示了猎户座棒光解离区（PDR）在 [CI] 609μm、C2H (4-3) 和 C18O (3-2) 发射线中的亚角秒分辨率 ALMA 拼接图，并辅以 H2 和芳香红外波段（AIB）发射的 JWST 图像。棒的前缘显示出由小尺度 H2 解离前沿（DFs）构成的非常起伏的结构。[CI] 609μm 发射峰非常靠近（约0.002 pc）主要的 H2 发射 DFs，表明存在气体密度梯度。这些 DFs 在 C2H 发射中也很明亮且非常相似，C2H 发射追踪了以极高 C2H 丰度为特征的“烃自由基峰”，丰度可达数个×10^-7。C2H 及相关烃自由基（如 CH3、CH2 和 CH）的高丰度可归因于由高温、C+ 和 C 的存在以及远紫外泵浦的 H2 的反应性所驱动的气相反应。烃自由基峰大致与 3.4/3.3μm AIB 强度比的最大值重合，该比值是 PAHs 中脂肪族与芳香族含量的代理指标。这意味着触发简单烃形成的条件也有利于带有脂肪族侧基的 PAHs 的形成（和存活），可能通过自下而上过程的贡献，其中丰富的烃自由基与 PAHs 在原位发生反应。在 DFs 前方，在原子 PDR 区（其中 [H]>>[H2]），AIB 发射最亮，但由于更强的远紫外场，小 PAHs 和碳质颗粒经历了光处理。我们在该区域检测到痕量 C2H，可能源于这些物质的光侵蚀。本研究提供了 PDR 中关键碳载体的化学分层的空间分辨视图。总体而言，自下而上和自上而下过程似乎都将简单烃分子与分子云中的 PAHs 联系起来；然而，确切的化学路径及其相对贡献仍有待量化。
英文摘要：
We present subarcsecond-resolution ALMA mosaics of the Orion Bar PDR in [CI] 609um, C2H (4-3), and C18O (3-2) emission lines complemented by JWST images of H2 and aromatic infrared band (AIB) emission. The rim of the Bar shows very corrugated structures made of small-scale H2 dissociation fronts (DFs). The [CI] 609 um emission peaks very close (~0.002 pc) to the main H2-emitting DFs, suggesting the presence of gas density gradients. These DFs are also bright and remarkably similar in C2H emission, which traces "hydrocarbon radical peaks" characterized by very high C2H abundances, reaching up to several x10^-7. The high abundance of C2H and of related hydrocarbon radicals, such as CH3, CH2, and CH, can be attributed to gas-phase reactions driven by elevated temperatures, the presence of C+ and C, and the reactivity of FUV-pumped H2. The hydrocarbon radical peaks roughly coincide with maxima of the 3.4/3.3 um AIB intensity ratio, a proxy for the aliphatic-to-aromatic content of PAHs. This implies that the conditions triggering the formation of simple hydrocarbons also favor the formation (and survival) of PAHs with aliphatic side groups, potentially via the contribution of bottom-up processes in which abundant hydrocarbon radicals react in situ with PAHs. Ahead of the DFs, in the atomic PDR zone (where [H]>>[H2]), the AIB emission is the brightest, but small PAHs and carbonaceous grains undergo photo-processing due to the stronger FUV field. Our detection of trace amounts of C2H in this zone may result from the photoerosion of these species. This study provides a spatially resolved view of the chemical stratification of key carbon carriers in a PDR. Overall, both bottom-up and top-down processes appear to link simple hydrocarbon molecules with PAHs in molecular clouds; however, the exact chemical pathways and their relative contributions remain to be quantified.
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