Atomic Oxygen Abundance in Molecular Clouds: Absorption toward Sagittarius B2

We have obtained high-resolution (~35 km s -1 ) spectra toward the molecular cloud Sgr B2 at 63 μm, the wavelength of the ground-state fine-structure line of atomic oxygen (O I), using the ISO -LWS instrument. Four separate velocity components are seen in the deconvolved spectrum, in absorption against the dust continuum emission of Sgr B2. Three of these components, corresponding to foreground clouds, are used to study the O I content of the cool molecular gas along the line of sight. In principle, the atomic oxygen that produces a particular velocity component could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in the UV illuminated surface layer (PDR) of a cloud, and in an atomic (H I) gas. For each of the three foreground clouds, we estimate, and subtract from the observed O I column density, the oxygen content of the H I gas, by scaling from a published high-resolution 21 cm spectrum. We find that the remaining O I column density is correlated with the observed 13 CO column density. From the slope of this correlation, an average [O I]/[ 13 CO] ratio of 270 ± 120 (3 σ) is derived, which corresponds to [O I]/[CO] ≈ 9 for a CO to 13 CO abundance ratio of 30. Assuming a 13 CO abundance of 1 × 10 -6 with respect to H nuclei, we derive an atomic oxygen abundance of 2.7 × 10 -4 in the dense gas phase, corresponding to a 15% oxygen depletion compared to the diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination of the PDR contribution to the O I column density. The PDR regions should contain O I but not 13 CO, and would thus be expected to produce an offset in the O I- 13 CO correlation. Our data do not show such an offset, suggesting that within our beam O I is spatially coexistent with the molecular gas, as traced by 13 CO. This may be a result of the inhomogeneous nature of the clouds.