Herschel Observations Reveal Anomalous Molecular Abundances toward the Galactic Center

Abstract

We report the Herschel detections of hydrogen fluoride (HF) and para-water (p-H₂O) in gas intercepting the sight lines to two well-studied molecular clouds in the vicinity of the Sgr A complex: G-0.02-0.07 (the "+50 km s^-1 cloud") and G-0.13-0.08 (the "+20 km s^-1 cloud"). Toward both sight lines, HF and water absorption components are detected over a wide range of velocities covering ~250 km s^-1. For all velocity components with V _LSR > -85 km s^-1, we find that the HF and water abundances are consistent with those measured toward other sight lines probing the Galactic disk gas. The velocity components with V _LSR <= -85 km s^-1, which are known to trace gas residing within ~200 pc of the Galactic center, however, exhibit water vapor abundances with respect to HF at least a factor three higher than those found in the Galactic disk gas. Comparison with CH data indicates that our observations are consistent with a picture where HF and a fraction of the H₂O absorption arise in diffuse molecular clouds showing Galactic disk-like abundances while the bulk of the water absorption arises in warmer (T >= 400 K) diffuse molecular gas for V _LSR <= -85 km s^-1. This diffuse Interstellar Medium (ISM) phase has also been recently revealed through observations of CO, HF, H⁺ ₃, and H₃O⁺ absorption toward other sight lines probing the Galactic center inner region.