DISCOVERY OF INTERSTELLAR HYDROGEN-FLUORIDE

We report the first detection of interstellar hydrogen fluoride, Using the Long Wavelength Spectrometer of the Infrared Space Observatory (I SO), we have detected the 121.6973 mu m J = 2-1 line of HF in absorpti on toward the far-infrared continuum source Sagittarius B2. The detect ion is statistically significant at the 13 sigma level. On the basis o f our model for the excitation of HF in Sgr B2, the observed line equi valent width of 1.0 nm implies a hydrogen fluoride abundance of simila r to 3 x 10(-10) relative to H-2. If the elemental abundance of fluori ne in Sgr B2 is the same as that in the solar system, then HF accounts for similar to 2% of the total number of fluorine nuclei. We expect h ydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomi c fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust g rains, Similar conclusions regarding depletion have previously been re ached for the case of chlorine in dense interstellar clouds. We also f ind evidence at a lower level of statistical significance (similar to 5 sigma) for an emission feature at the expected position of the 4(32) -4(23) 121.7219 mu m line of water The emission-line equivalent width of 0.5 nm for the water feature is consistent with the water abundance of 5 x 10(-6) relative to H-2 that has been inferred previously from observations of the hot core of Sgr B2.