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

We report the detection of strong absorption by interstellar hydrogen fluoride 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, to observe the 1232.4763 GHz J= 1–0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6–0.4 at LSR velocities in the range –10 to –3 km s-1, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km s-1. The spectrum is similar to that of the 1113.3430 GHz 111–000transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H2O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6×1014cm-2on the HF column density along the sight-line to G10.6–0.4. Our lower limit on the HF abundance, 6×10-9relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ~30% and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that – because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen – HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions.