Origin of hydrogen fluoride emission in the Orion Bar: An excellent
  tracer for CO-dark H2 gas clouds
  The hydrogen fluoride (HF) molecule is seen in absorption in the interstellar
medium (ISM) along many lines of sight. Surprisingly, it is observed in
emission toward the Orion Bar, which is an interface between the ionized region
around the Orion Trapezium stars and the Orion molecular cloud. We aim to
understand the origin of HF emission in the Orion Bar by comparing its spatial
distribution with other tracers. We examine three mechanisms to explain the HF
emission: thermal excitation, radiative dust pumping, and chemical pumping. We
used a Herschel/HIFI strip map of the HF J = 1-0 line, covering 0.5' by 1.5'
that is oriented perpendicular to the Orion Bar. We used the RADEX non-local
thermodynamic equilibrium (non-LTE) code to construct the HF column density
map. We use the Meudon PDR code to explain the morphology of HF. The bulk of
the HF emission at 10 km s^-1 emerges from the CO-dark molecular gas that
separates the ionization front from the molecular gas that is deeper in the
Orion Bar. The excitation of HF is caused mainly by collisions with H2 at a
density of 10^5 cm^-3 together with a small contribution of electrons in the
interclump gas of the Orion Bar. Infrared pumping and chemical pumping are not
important. We conclude that the HF J = 1-0 line traces CO-dark molecular gas.
Similarly, bright photodissociation regions associated with massive star
formation may be responsible for the HF emission observed toward active
galactic nuclei.
