Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion KL

ABSTRACT We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules ("weeds"), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 10 13 cm −2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H 2 column density within the low-velocity molecular outflow, we obtain a lower limit of ∼ 1.6 × 10 −10 for the HF abundance relative to hydrogen nuclei, corresponding to ∼ 0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J=2-1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 × 10 −9 derived by Neufeld et al