Spatial distribution of FIR rotationally excited CH+ and OH emission
  lines in the Orion Bar PDR
  The abundance of CH+ and OH and excitation are predicted to be enhanced by
the presence of vibrationally excited H2 or hot gas (~500-1000 K) in PDRs with
high incident FUV radiation field. The excitation may also originate in dense
gas (>10^5 cm-3) followed by nonreactive collisions. Previous observations
suggest that the CH+ and OH correlate with dense and warm gas, and formation
pumping contributes to CH+ excitation. We examine the spatial distribution of
the CH+ and OH emission in the Orion Bar to establish their physical origin and
main formation and excitation mechanisms. We present spatially sampled maps of
the CH+ J=3-2 transition at 119.8 {\mu}m and the OH {\Lambda}-doublet at 84
{\mu}m 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. We assess
the spatial variation of CH+ J=2-1 velocity-resolved line profile observed with
Herschel (HIFI). The OH and CH+ lines correlate well with the high-J CO
emission and delineate the warm and dense molecular region. While similar, the
differences in the CH+ and OH morphologies indicate that CH+ formation and
excitation are related to the observed vibrationally excited H2. This indicates
that formation pumping contributes to the excitation of CH+. Interestingly, the
peak of the rotationally excited OH 84 {\mu}m emission coincides with a bright
young object, proplyd 244-440, which shows that OH can be an excellent tracer
of UV-irradiated dense gas. 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 {\Lambda}-doublet at 84 {\mu}m
is mainly sensitive to the temperature and density.
