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

Abstract

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 <i>Herschel<i/>'s HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz <i>J<i/> = 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<sup>-1<sup/>, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km s<sup>-1<sup/>. The spectrum is similar to that of the 1113.3430 GHz 1<sub>11<sub/>–0<sub>00<sub/> transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H<sub>2<sub/>O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6×10<sup>14<sup/> cm<sup>-2<sup/> on the HF column density along the sight-line to G10.6–0.4. Our lower limit on the HF abundance, 6×10<sup>-9<sup/> relative 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.