INFRARED SPACE OBSERVATORY LONG WAVELENGTH SPECTROMETER OBSERVATIONS OF C+ AND O0 LINES IN ABSORPTION TOWARD SAGITTARIUS B21

High spectral resolution Fabry-Pérot observations of the [O i] 63.2 and 145.5 lm and [C ii] 157.7 lm 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 lm and the [C ii] 157.7 lm lines are detected in absorption. The upper state level of atomic oxygen at 145.5 lm is in emission. Whereas the [O i] 63.2 lm line is seen in absorption over the entire wavelength range 200 to 100 km s1, the [C ii] 157.7 lm line displays a more complex profile: absorption occurs at velocities less than 20 km s1, and emission comes from the Sgr B2 complex at velocities greater than 20 km s1. 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 cm2 and 3 1019 cm2, 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 for