EDP Sciences
Free access
Volume 499, Number 3, June I 2009
Page(s) 799 - 809
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361/200811392
Published online 08 April 2009
A&A 499, 799-809 (2009)
DOI: 10.1051/0004-6361/200811392

Oxygen in dense interstellar gas

The oxygen abundance of the star forming core $\rho$ Ophiuchi A
R. Liseau and K. Justtanont

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

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 $\rho$ Oph A to derive the temperatures and column densities of H2 at the locations of ISO-LWS observations of two [O I] 3PJ lines. The intensity ratios of the (J = 1–2) 63 $\mu$m to (J = 0–1) 145 $\mu$m 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 $\mu$m and [O I] 145 $\mu$m 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 $\times$ 10-5.
Conclusions. As option (1) disregards valid [O I] 145 $\mu$m 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.

Key words: ISM: abundances -- ISM: molecules -- ISM: dust, extinction -- ISM: clouds -- ISM: individual objects: $\rho$ Ophiuchi A -- ISM: individual objects: VLA 1623

© ESO 2009