2017A&A...599A..20P

Context. 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 H₂ or hot gas (∼500-1000K) in photodissociation regions (PDRs) with high incident far-ultraviolet (FUV) radiation field. The excitation may also originate in dense gas (>10⁵cm^–3) followed by nonreactive collisions with H₂, H, and electrons. Previous observations of the Orion Bar suggest that the rotationally excited CH⁺ and OH correlate with the excited CO, which is a tracer of dense and warm gas, and that formation pumping contributes to CH⁺ excitation.
Aims. Our goal is to examine the spatial distribution of the rotationally excited CH⁺ and OH emission lines in the Orion Bar to establish their physical origin and main formation and excitation mechanisms.
Methods. 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''x110'' with Herschel/PACS. We compare the spatial distribution of these molecules with those of their chemical precursors, C⁺, O and H₂, and tracers of warm and dense gas (high-J CO). We assess the spatial variation of the CH⁺ J=2-1 velocity-resolved line profile at 1669GHz with Herschel/HIFI spectrometer observations.
Results. 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 H₂. 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 84µm emission coincides with a bright young object, proplyd 244-440, which shows that OH can be an excellent tracer of UV-irradiated dense gas.
Conclusions. 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 84µm is mainly sensitive to the temperature and density.