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
  The abundances of interstellar CH+ and SH+ are not well understood as their
most likely formation channels are highly endothermic. Using data from
Herschel, we study the formation of CH+ and SH+ in a typical high
UV-illumination photon-dominated region (PDR), the Orion Bar. Herschel/HIFI
provides velocity-resolved data of CH+ 1-0 and 2-1 and three hyperfine
transitions of SH+. Herschel/PACS provides information on the excitation and
spatial distribution of CH+ (up to J=6-5). The widths of the CH+ 2-1 and 1-0
transitions are of ~5 km/s, significantly broader than the typical width of
dense gas tracers in the Orion Bar (2-3 km/s) and are comparable to the width
of tracers of 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,
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 500-1000 K. We have also detected the 5-4 transition of
CF+ (FWHM=1.9 km/s) with an intensity that is consistent with previous
observations of lower-J CF+ transitions toward the Orion Bar. A 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. H2 vibrational excitation is the most likely explanation of
SH+ formation as well. The abundance ratios of CH+ and SH+ trace the
destruction paths of these ions, and through that, indirectly, the ratios of H,
H2 and electron abundances as a function of depth into the cloud.
