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The hydrogen fluoride (HF) molecule is seen in absorption in the interstellarmedium (ISM) along many lines of sight. Surprisingly, it is observed inemission toward the Orion Bar, which is an interface between the ionized regionaround the Orion Trapezium stars and the Orion molecular cloud. We aim tounderstand the origin of HF emission in the Orion Bar by comparing its spatialdistribution with other tracers. We examine three mechanisms to explain the HFemission: thermal excitation, radiative dust pumping, and chemical pumping. Weused 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-localthermodynamic equilibrium (non-LTE) code to construct the HF column densitymap. We use the Meudon PDR code to explain the morphology of HF. The bulk ofthe HF emission at 10 km s^-1 emerges from the CO-dark molecular gas thatseparates the ionization front from the molecular gas that is deeper in theOrion Bar. The excitation of HF is caused mainly by collisions with H2 at adensity of 10^5 cm^-3 together with a small contribution of electrons in theinterclump gas of the Orion Bar. Infrared pumping and chemical pumping are notimportant. We conclude that the HF J = 1-0 line traces CO-dark molecular gas.Similarly, bright photodissociation regions associated with massive starformation may be responsible for the HF emission observed toward activegalactic nuclei.

Origin of hydrogen fluoride emission in the Orion Bar: An excellent tracer for CO-dark H2 gas clouds