Discovery of Interstellar Hydrogen Fluoride

Abstract

We report the first detection of interstellar hydrogen fluoride. Using the Long Wavelength Spectrometer of the Infrared Space Observatory ( ISO ), we have detected the 121.6973 μm J =2-1 line of HF in absorption toward the far-infrared continuum source Sagittarius B2. The detection is statistically significant at the 13 σ level. On the basis of our model for the excitation of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a hydrogen fluoride abundance of ~3 × 10 -10 relative to H 2 . If the elemental abundance of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts for ~2% of the total number of fluorine nuclei. We expect hydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomic fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust grains. Similar conclusions regarding depletion have previously been reached for the case of chlorine in dense interstellar clouds. We also find evidence at a lower level of statistical significance (~5 σ) for an emission feature at the expected position of the 4 32 -4 23 121.7219 μm line of water. The emission-line equivalent width of 0.5 nm for the water feature is consistent with the water abundance of 5 × 10 −6 relative to H 2 that has been inferred previously from observations of the hot core of Sgr B2.