Oxygen in dense interstellar gas^*

The oxygen abundance of the star forming core ρ Ophiuchi A

Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden e-mail: [rene.liseau;kay.justtanont]@chalmers.se

Received: 21 November 2008
Accepted: 19 March 2009

Abstract

Context. Oxygen is the third most abundant element in the universe, but its chemistry in the interstellar medium is still not understood well.

Aims. To critically examine the entire oxygen budget, we initially attempt to estimate the abundance of atomic oxygen, O, in the only region where molecular oxygen, O₂, has been detected to date.

Methods. We analysed ISOCAM-CVF spectral image data toward ρ Oph A to derive the temperatures and column densities of H₂ at the locations of ISO-LWS observations of two [O I] ³P_J lines. The intensity ratios of the (J = 1–2) 63 μm to (J = 0–1) 145 μm lines largely exceed ten, attesting to these lines being optically thin. This is confirmed by radiative transfer calculations, making these lines suitable for abundance determinations. For that purpose, we calculated line strengths and compared 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 essentially leads to 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; instead, 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.