Tentative detection of molecular oxygen in the ρ Ophiuchi cloud

We report the tentative detection of molecular oxygen in the interstellar medium. Deep integrations using the Submillimeter Wave Astronomy (SWAS) satellite of the rho Oph A cloud have resulted in a detection of an emission feature consistent with the N-J = 3(3) --> 1(2) transition of O-2 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 O-2 relative to H-2 in this high-velocity gas is approximately 10(-5). The fractional abundance of ortho-H2O in this material is 8 x 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 O-2 abundance in cloud cores previously reported [X(O-2) less than or equal to few x 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 10(5) yr, the O-2 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 O-2 drops by several orders of magnitude, with a significant fraction of oxygen remaining in atomic form.