Infrared Space Observatory Long Wavelength Spectrometer Observations of C⁺ and O⁰ Lines in Absorption toward Sagittarius B2^*

High spectral resolution Fabry-Pérot observations of the [O I] 63.2 and 145.5 μm and [C II] 157.7 μm fine-structure lines are presented for the center of the Sagittarius B2 complex (Sgr B2). The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). Both the [O I] 63.2 μm and the [C II] 157.7 μm lines are detected in absorption. The upper state level of atomic oxygen at 145.5 μm is in emission. Whereas the [O I] 63.2 μm line is seen in absorption over the entire wavelength range -200 to 100 km s^-1, the [C II] 157.7 μm line displays a more complex profile: absorption occurs at velocities less than 20 km s^-1, and emission comes from the Sgr B2 complex at velocities greater than 20 km s^-1. Using observations of the CO isotopes and of the H I lines, absorption components can be associated with many clouds along the Sgr B2 line of sight. From these data, we were able to disentangle three different layers that contain atomic oxygen. These layers, as predicted by photodissociation region models, are characterized by different forms of carbon in the gas phase, i.e., the C⁺ external layer, the C⁺ to C⁰ transition, and the CO internal layer. We derive lower limits for the column densities of atomic carbon and oxygen of the order of ~10¹⁸ cm^-2 and 3 × 10¹⁹ cm^-2, respectively. An O⁰/CO ratio of around 2.5 is computed in the internal cores of the clouds lying along the line of sight, which means that ~70% of gaseous oxygen is in the atomic form and not locked into CO. The fact that the [C II] 157.7 μm line is detected in absorption implies that the main cooling line of the interstellar medium can be optically thick, especially in the direction of large star-forming complexes or in the nuclei of galaxies. This could partially account for the deficiency in the [C II] 157.7 μm line that recently has been found toward infrared-bright galaxies in ISO data.