Parikka A, Habart E, Bernard-Salas J, Goicoechea J R, Abergel A, Pilleri P, Dartois E, Joblin C, Gerin M, Godard B
Institut d'Astrophysique Spatiale, Université Paris-Saclay, 91405 Orsay Cedex, France; I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany.
Institut d'Astrophysique Spatiale, Université Paris-Saclay, 91405 Orsay Cedex, France.
Astron Astrophys. 2017 Mar;599. doi: 10.1051/0004-6361/201629445. Epub 2017 Feb 21.
The methylidyne cation (CH) and hydroxyl (OH) are key molecules in the warm interstellar chemistry, but their formation and excitation mechanisms are not well understood. Their abundance and excitation are predicted to be enhanced by the presence of vibrationally excited H or hot gas (~500-1000 K) in photodissociation regions with high incident FUV radiation field. The excitation may also originate in dense gas (> 10 cm) followed by nonreactive collisions with H, H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH and OH correlate with the excited CO, a tracer of dense and warm gas, and formation pumping contributes to CH excitation.
Our goal is to examine the spatial distribution of the rotationally excited CH and OH emission lines in the Orion Bar in order to establish their physical origin and main formation and excitation mechanisms.
We present spatially sampled maps of the CH J=3-2 transition at 119.8 µm and the OH Λ-doublet at 84 µm in the Orion Bar over an area of 110″×110″ with (PACS). We compare the spatial distribution of these molecules with those of their chemical precursors, C, O and H, and tracers of warm and dense gas (high-J CO). We assess the spatial variation of CH J=2-1 velocity-resolved line profile at 1669 GHz with HIFI spectrometer observations.
The OH and especially CH lines correlate well with the high-J CO emission and delineate the warm and dense molecular region at the edge of the Bar. While notably similar, the differences in the CH and OH morphologies indicate that CH formation and excitation are strongly related to the observed vibrationally excited H. This, together with the observed broad CH line widths, indicates that formation pumping contributes to the excitation of this reactive molecular ion. Interestingly, the peak of the rotationally excited OH 84 µm emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas.
The spatial distribution of CH and OH revealed in our maps is consistent with previous modeling studies. Both formation pumping and nonreactive collisions in a UV-irradiated dense gas are important CH J=3-2 excitation processes. The excitation of the OH Λ-doublet at 84 µm is mainly sensitive to the temperature and density.
次甲基阳离子（CH）和羟基（OH）是温暖星际化学中的关键分子，但其形成和激发机制尚未完全明晰。在具有高入射远紫外辐射场的光解离区域中，振动激发的H或热气体（约500 - 1000 K）的存在预计会增强它们的丰度和激发。激发也可能起源于高密度气体（> 10 cm），随后与H、H和电子发生非反应性碰撞。先前对猎户座棒状星云的观测表明，转动激发的CH和OH与激发的CO相关，CO是致密温暖气体的示踪剂，形成抽运有助于CH的激发。
我们的目标是研究猎户座棒状星云中转动激发的CH和OH发射线的空间分布，以确定它们的物理起源以及主要的形成和激发机制。
我们利用（PACS）在猎户座棒状星云110″×110″的区域内，呈现了119.8 µm处CH J = 3 - 2跃迁和84 µm处OH Λ双重线的空间采样图。我们将这些分子的空间分布与其化学前体C、O和H以及温暖致密气体（高J CO）的示踪剂的分布进行比较。我们利用HIFI光谱仪观测评估了1669 GHz处CH J = 2 - 1速度分辨线轮廓的空间变化。
OH线，尤其是CH线与高J CO发射很好地相关，并勾勒出棒状星云边缘温暖致密的分子区域。虽然明显相似，但CH和OH形态的差异表明CH的形成和激发与观测到的振动激发的H密切相关。这与观测到的较宽的CH线宽一起，表明形成抽运有助于这种反应性分子离子的激发。有趣的是，转动激发的84 µm OH发射的峰值与一个明亮的年轻天体，原行星盘244 - 440重合，这表明OH可以是紫外线照射的致密气体极好的示踪剂。
我们的图中揭示的CH和OH的空间分布与先前的模型研究一致。在紫外线照射的致密气体中的形成抽运和非反应性碰撞都是重要的CH J = 3 - 2激发过程。84 µm处OH Λ双重线的激发主要对温度和密度敏感。
