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We report the detection of strong absorption by interstellar hydrogenfluoride along the sight-line to the submillimeter continuum source G10.6-0.4(W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, toobserve the 1232.4763 GHz J=1-0 HF transition in the upper sideband of the Band5a receiver. The resultant spectrum shows weak HF emission from G10.6-0.4 atLSR velocities in the range -10 to -3 km/s, accompanied by strong absorption byforeground material at LSR velocities in the range 15 to 50 km/s. The spectrumis similar to that of the 1113.3430 GHz 1(11)-0(00) transition of para-water,although at some frequencies the HF (hydrogen fluoride) optical depth clearlyexceeds that of para-H2O. The optically-thick HF absorption that we haveobserved places a conservative lower limit of 1.6E+14 cm-2 on the HF columndensity along the sight-line to G10.6-0.4. Our lower limit on the HF abundance,6E-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts forbetween ~ 30 and 100% of the fluorine nuclei in the gas phase along thissight-line. This observation corroborates theoretical predictions that -because the unique thermochemistry of fluorine permits the exothermic reactionof F atoms with molecular hydrogen - HF will be the dominant reservoir ofinterstellar fluorine under a wide range of conditions.

Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)