The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+ The effect of high electron density and vibrationally excited H-2 in a warm PDR surface

Journal article, 2013

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 H-2 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+ 1(2)-0(1). 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 similar to 5 kms(-1), significantly broader than the typical width of dense gas tracers in the Orion Bar (similar to 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 similar to 3 kms(-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 (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 similar to 1.9 kms(-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 H-2 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 H-2 vibrational excitation. The abundance ratios of CH+ and SH+ trace the destruction paths of these ions, and indirectly, the ratios of H, H-2, and electron abundances as a function of depth into the cloud.