Atomic Oxygen in Molecular Clouds? High-Resolution Spectroscopy of the [O I] 63 Micron Line toward DR 21

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We report the first high-resolution spectra of the [O I] 63 μm fine-structure line toward DR 21. The observations were made from NASA's Kuiper Airborne Observatory using the MPE/UCB imaging spectrometer FIFI. The spectra show a pronounced dip, which may be due to absorption by a foreground molecular cloud, seen against the broad [O I] emission from DR 21. In this case we derive a column density of cold, atomic oxygen of 5 × 1018 cm-2, corresponding to a relative abundance of atomic oxygen of [O]/[H] 6 × 10-4. Therefore, most of the gas-phase oxygen in this cloud would be in atomic form. This result is in contrast to predictions by common chemistry models for a steady state cloud and may support models which predict a high abundance of atomic oxygen in dark cores of molecular clouds.