Herschel Observations Reveal Anomalous Molecular Abundances toward the Galactic Center

We report the Herschel detections of hydrogen fluoride (HF) and para-water (p-H_(2)O) 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 H_(2)O 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 H_(3)O^+ absorption toward other sight lines probing the Galactic center inner region