2002ApJ...576..814G

We report the tentative detection of molecular oxygen in the interstellar medium. Deep integrations using the Submillimeter Wave Astronomy (SWAS) satellite of the ρ Oph A cloud have resulted in a detection of an emission feature consistent with the N_J=3₃⟶1₂transition of O₂ at a local standard of rest velocity of 6.0 km.s^–1. The line width and velocity are suggestive of the redshifted wing emission seen in species that trace the molecular outflows seen in this region. The fractional abundance of O₂relative to H₂in this high-velocity gas is approximately 10^–5. The fractional abundance of ortho-H₂O in this material is 8x10^–8, somewhat higher than found in quiescent material by earlier SWAS observations but less than that found in younger outflows. We suggest a unified scenario that explains the low O₂abundance in cloud cores previously reported [X(O₂)≤fewx10^–7], along with the much enhanced abundance in the outflow region. In this picture, quiescent clouds are characterized by significant depletion of gas-phase oxygen, which ends up largely as water ice on the dust grains. A shock associated with the outflow results in almost all oxygen being processed into gas-phase water. In the cool postshock gas, the standard gas-phase chemistry reasserts itself, and after an elapsed time of a few times 10⁵ yr, the O₂abundance has increased to the point that this species is one of the main repositories of oxygen atoms. At the same time, the gas-phase water abundance drops drastically, in agreement with the SWAS observations that we report here. On a somewhat longer timescale, depletion again dominates and the gas-phase abundance of O₂drops by several orders of magnitude, with a significant fraction of oxygen remaining in atomic form.