Tentative Detection of Molecular Oxygen in the ρ Ophiuchi Cloud

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Abstract
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 NJ = 33 → 12 transition 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 O2 relative to H2 in this high-velocity gas is approximately 10-5. The fractional abundance of ortho-H2O in this material is 8 × 10-8, 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 O2 abundance in cloud cores previously reported [X(O2) ≤ few × 10-7], 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 O2 abundance 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 O2 drops by several orders of magnitude, with a significant fraction of oxygen remaining in atomic form.