2013A&A...550A..96N

The abundances of interstellar CH⁺ and SH⁺ are not well understood as their most likely formation channels are highly endothermic. Several mechanisms have been proposed to overcome the high activation barriers, including shocks, turbulence, and H₂ vibrational excitation. Using data from the Herschel Space Observatory, we studied the formation of ions, in particular CH⁺ and SH⁺ in a typical high UV-illumination warm and dense photon-dominated region (PDR), the Orion Bar. The HIFI instrument on board Herschel provides velocity-resolved line profiles of CH⁺ 1-0 and 2-1 and three hyperfine transitions of SH⁺ 1₂-0₁. The PACS instrument provides information on the excitation and spatial distribution of CH⁺ by extending the observed CH⁺ transitions up to J=6-5. We compared the observed line intensities to the predictions of radiative transfer and PDR codes. All CH⁺, SH⁺, and CF⁺ lines analyzed in this paper are seen in emission. The widths of the CH⁺ 2-1 and 1-0 transitions are of ∼5km/s, significantly broader than the typical width of dense gas tracers in the Orion Bar (∼2-3km/s) and are comparable to the width of species that trace the interclump medium such as C⁺ and HF. The detected SH⁺ transitions are narrower compared to CH⁺ and have line widths of ∼3km/s, indicating that SH⁺ emission mainly originates in denser condensations. Non-LTE radiative transfer models show that electron collisions affect the excitation of CH⁺ and SH⁺ and that reactive collisions need to be taken into account to calculate the excitation of CH⁺. Comparison to PDR models shows that CH⁺ and SH⁺ are tracers of the warm surface region (A_V<1.5) of the PDR with temperatures between 500 and 1000 K. We have also detected the 5-4 transition of CF⁺ at a width of ∼1.9km/s, consistent with the width of dense gas tracers. The intensity of the CF⁺ 5-4 transition is consistent with previous observations of lower-J transitions toward the Orion Bar. An analytic approximation and a numerical comparison to PDR models indicate that the internal vibrational energy of H₂ can explain the formation of CH⁺ for typical physical conditions in the Orion Bar near the ionization front. The formation of SH⁺ is also likely to be explained by H₂ vibrational excitation. The abundance ratios of CH⁺ and SH⁺ trace the destruction paths of these ions, and indirectly, the ratios of H, H₂, and electron abundances as a function of depth into the cloud.