Atomic oxygen abundance in molecular clouds: Absorption toward SagittariusB2

We have obtained high-resolution (similar to 35 km s(-1)) spectra toward th e molecular cloud Sgr B2 at 63 mum, 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 absor ption against the dust continuum emission of Sgr B2. Three of these compone nts, 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, o r all, of three physically distinct regions : inside a dense molecular clou d, 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 subtrac t 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 th e remaining O I column density is correlated with the observed (CO)-C-13 co lumn density. From the slope of this correlation, an average [O I]/[(CO)-C- 13] ratio of 270 +/- 120 (3 sigma) is derived, which corresponds to [O I]/[ CO] approximate to 9 for a CO to (CO)-C-13 abundance ratio of 30. Assuming a (CO)-C-13 abundance of 1 x 10(-6) with respect to H nuclei, we derive an atomic oxygen abundance of 2.7 x 10(-4) in the dense gas phase, correspondi ng to a 15% oxygen depletion compared to the diffuse ISM in our Galactic ne ighborhood. The presence of multiple, spectrally resolved velocity componen ts in the Sgr B2 absorption spectrum allows, for the first time, a direct d etermination of the PDR contribution to the O I column density. The PDR reg ions should contain O I but not (CO)-C-13, and would thus be expected to pr oduce an offset in the O I-(CO)-C-13 correlation. Our data do not show such an offset, suggesting that within our beam O I is spatially coexistent wit h the molecular gas, as traced by (CO)-C-13. This may be a result of the in homogeneous nature of the clouds.