发表机构
Instituto de Física Fundamental (CSIC); Institut de Radioastronomie Millimétrique; LUX, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités; Univ. Bordeaux, CNRS, Laboratoire d’Astrophysique de Bordeaux, UMR5804（基础物理研究所; 毫米波射电天文学研究所; 巴黎天文台; 波尔多大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
利用ALMA亚角秒成像观测猎户座棒状结构中的HCN和CN发射，发现其峰值靠近离解前沿，揭示紫外驱动化学在致密环境中形成暖氮气体并示踪密度梯度。
AI 中文摘要
由于高临界密度，转动激发的HCN和CN谱线常被用作气体密度的示踪剂。ALMA凭借其高光谱和高空间分辨率，为研究HCN和CN发射的空间形态和运动学提供了独特途径，从而可以研究分子云边缘的亚结构。我们展示了猎户座棒状结构中HCN 4-3和CN 3-2谱线的新亚角秒成像，以探测其密度结构。我们研究了它们的空间分布，并将其与光致离解区（PDR）内其他现有示踪剂进行了比较。我们将这些数据与IRAM 30米和赫歇尔望远镜观测到的多个J谱线相结合，以研究离解前沿（DF）附近的HCN和CN激发。我们发现，与C2H和H2相比，HCN和CN发射在更受紫外屏蔽的云方向上空间延展更广。然而，两者都在离解前沿附近达到峰值，这与之前较低角分辨率观测结果不一致。这种形态可以用增强的紫外驱动化学来解释。在致密且受辐照的环境中，HCN通过H2+CN反应有效形成。因此，其发射也可以示踪离解前沿附近的较暖气体。在原子PDR中观测到CN发射，由于H2丰度极低，稳态气相化学无法解释这一现象。我们将其解释为含CN多环芳烃（PAHs）的光化学处理。当将观测结果与RADEX和默东PDR代码模型比较时，我们推导出离解前沿附近的高密度。最后，我们没有发现小尺度高密度团块的证据，并认为观测到的密度梯度可以解释不同示踪剂之间激发的差异。我们在HCN发射中分辨出两个速度分量：10.5 km/s（棒状结构的速度）和11.5 km/s。在这个红移发射分量中观测到的HCN高度结构化发射，更符合紫外诱导激波压缩气体的特征，而非团块。
英文摘要
Rotationally excited HCN and CN lines, due to their high critical densities, are often used as tracers of gas densities. ALMA, with its high spectral and spatial resolution, provides unique access to the spatial morphology and kinematics of HCN and CN emission to study the substructures of the molecular cloud edges. We present new sub-arcsecond imaging of HCN 4-3 and CN 3-2 lines to probe the density structure in the Orion Bar. We study their spatial distribution and compare them to other existing tracers inside the PDR. We complemented these data with multiple J lines observed with the IRAM 30m and Herschel telescopes to study HCN and CN excitation near the dissociation front (DF). We find that HCN and CN emission are more spatially extended toward the more UV-shielded cloud than C2H and H2. However, they both peak very close to the DF, in disagreement with previous lower-angular-resolution observations. This morphology can be explained by the enhanced UV-driven chemistry. In dense and irradiated environments, HCN is efficiently formed via H2+CN. Hence, its emission can also probe warmer gas near the DF. CN emission is observed in the atomic PDR, not accounted by stationary gas-phase chemistry due to the very low H2 abundances. We interpret this as photoprocessing of CN-bearing PAHs. We derive high densities near the DF when comparing observations with RADEX and Meudon PDR Code models. Finally, we do not find evidence of small-scale high-density clumps and argue that the observed density gradient could explain the variation of excitation between the different tracers. We resolve two velocity components in HCN emission: 10.5 km/s (velocity of the Bar) and 11.5 km/s. The very structured emission of HCN observed in this redshifted emission component is more compatible with the signature of the propagation of a UV-induced shock compressing the gas than clumps.
CommentsAccepted in A&A on the 16/09/2026
