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 VLSR > -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 VLSR ≤ -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 VLSR ≤ -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. © 2013. The American Astronomical Society. All rights reserved.