Oxygen in dense interstellar gas ? The oxygen abundance of the star forming core ρ Oph A

Context. Oxygen is the third most abundant element in the universe, but its chemistry in the interstellar medium is still not well understood. Aims. In order to critically examine the entire oxygen budget, we attempt here initially to estimate the abundance of atomic oxygen, O, in the only one region, where molecular oxygen, O2, has been detected to date. Methods. We analyse ISOCAM-CVF spectral image data toward ρOph A to derive the temperatures and column densities of H2 at the locations of ISO-LWS observations of two [O I] PJ lines. The intensity ratios of the (J=1-2) 63 μm to (J=0-1) 145 μm lines largely exceed ten, attesting to the fact that these lines are optically thin. This is confirmed by radiative transfer calculations, making these lines suitable for abundance determinations. For that purpose, we calculate line strengths and compare them to the LWS observations. Results. Excess [O I] emission is observed to be associated with the molecular outflow from VLA 1623. For this region, we determine the physical parameters, T and N(H2), from the CAM observations and the gas density, n(H2), is determined from the flux ratio of the [O i] 63 μm and [O i] 145 μm lines. For the oxygen abundance, our analysis leads to essentially three possibilities: (1) Extended low density gas with standard ISM O-abundance, (2) Compact high density gas with standard ISM O-abundance and (3) Extended high density gas with reduced oxygen abundance, [O/H] ∼ 2 × 10−5. Conclusions. As option (1) disregards valid [O i] 145 μm data, we do not find it very compelling; we favour option (3), as lower abundances are expected as a result of chemical cloud evolution, but we are not able to dismiss option (2) entirely. Observations at higher angular resolution than offered by the LWS are required to decide between these possibilities.