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Title

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

Creator

Neufeld , D. A.; Sonnentrucker , P.; Phillips , T. G.; Lis , D. C.; De Luca , M.; Goicoechea , J. R.; Black , J. H.; Gerin , M.; Bell , T.; Boulanger , F.; Cernicharo , J.; Coutens , A.; Dartois , E.; Kazmierczak , M.; Encrenaz , P.; Falgarone , E.; Geballe , T. R.; Giesen , T.; Godard , B.; Goldsmith , P. F.; Gry , C.; Gupta , H. ,; Hennebelle , P.; Herbst , E.; Hily-Blant , P.; Joblin , C.; Kolos , R.; Krelowski , J.; Martin-Pintado , J.; Menten , K. M.; Monje , R.; Mookerjea , B.; Pearson , Justin; Perault , M.; Persson , C.; Plume , R.; Salez , M.; Schlemmer , S.; Schmidt , M.; Stutzki , J.; Teyssier , D.; Vastel , C.; Yu , S.; Cais , Ph.; Caux , E.; Liseau , R.; Morris , P.; Planesas , P.

Description

International audience

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, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km/s. The spectrum is 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 clearly exceeds that of para-H2O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6E+14 cm-2 on the HF column density 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 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.