The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+. The effect of high electron density and vibrationally excited H2 in a warm PDR surface
Context. 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.
Aims: 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.
Methods: 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.
Results: 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 ~5 km s-1, significantly broader than the typical width of dense gas tracers in the Orion Bar (~2-3 km s-1) 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 ~3 km s-1, 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 (AV < 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 ~1.9 km s-1, 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.
Conclusions: 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.