Discovery of Interstellar Hydrogen Fluoride

We report the first detection of interstellar hydrogen fluoride. Using theLong Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO), wehave detected the 121.6973 micron J = 2 - 1 line of HF in absorption toward thefar-infrared continuum source Sagittarius B2. The detection is statisticallysignificant at the 13 sigma level. On the basis of our model for the excitationof HF in Sgr B2, the observed line equivalent width of 1.0 nm implies ahydrogen fluoride abundance of 3E-10 relative to H2. If the elemental abundanceof fluorine in Sgr B2 is the same as that in the solar system, then HF accountsfor ~ 2% of the total number of fluorine nuclei. We expect hydrogen fluoride tobe the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formedrapidly in exothermic reactions of atomic fluorine with either water ormolecular hydrogen; thus the measured HF abundance suggests a substantialdepletion of fluorine onto dust grains. Similar conclusions regarding depletionhave previously been reached for the case of chlorine in dense interstellarclouds. We also find evidence at a lower level of statistical significance (~ 5sigma) for an emission feature at the expected position of the 4(3,2)-4(2,3)121.7219 micron line of water. The emission line equivalent width of 0.5 nm forthe water feature is consistent with the water abundance of 5E-6 relative to H2that has been inferred previously from observations of the hot core of Sgr B2.Comment: 11 pages (AASTeX using aaspp4.sty) plus 2 figures; to appear in ApJ Letter