Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6-0.4 (W31C)
Menten, K. M.; Perault, M.; Boulanger, F.; Falgarone, E.; Liseau, R.; Krełowski, J.; Godard, B.; Martín-Pintado, J.; Morris, P.; Teyssier, D.; Planesas, P.; Lis, D. C.; Phillips, T. G.; Caux, E.; Cernicharo, J.; Encrenaz, P.; Gerin, M.; Goicoechea, J. R.; Goldsmith, P. F.; Gupta, H.; Herbst, E.; Joblin, C.; Neufeld, D. A.; Plume, R.; Schlemmer, S.; Stutzki, J.; Vastel, C.; Yu, S.; Schmidt, M.; Sonnentrucker, P.; Mookerjea, B.; Hennebelle, P.; Black, J. H.; Dartois, E.; Pearson, J.; Bell, T.; Hily-Blant, P.; de Luca, M.; Geballe, T. R.; Gry, C.; Coutens, A.; Kazmierczak, M.; Giesen, T.; Kołos, R.; Monje, R.; Persson, C.; Salez, M.; Cais, P.
United States, France, Spain, Sweden, India, Poland, Germany, Canada, Chile
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 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 111-000 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 1.6×1014 cm-2 on the HF column density along the sight-line to G10.6-0.4. Our lower limit on the HF abundance, 6×10-9 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.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.