Molecular Oxygen in the Interstellar Medium

Séminaire IPAG de Paul F. Goldsmith (JPL, Caltech), jeudi 14 juin 2012 à 11h00, IPAG seminar room

Models of the gas-phase chemistry in dense clouds predict molecular oxygen (O2) to be almost as abundant as carbon monoxide (CO). Searches for Galactic O2 carried out with the SWAS and Odin spacecraft have yielded upper limits on the abundance of molecular oxygen typically 2 orders of magnitude below those predicted by gas-phase models. A variety of explanations have been proposed to explain this low abundance. Some of these are based on depletion of atomic oxygen onto dust grains, resulting in incorporation of this species into water that remains on the grain surface. Other models involve circulation of material between UV-irradiated and well-shielded regions, and highly clumpy cloud structure. The Herschel Open Time Key Project ``HOP’’ (Herschel Oxygen Project) addresses this important problem in astrochemistry, exploiting the high angular resolution and sensitivity of the Herschel HIFI instrument to observe 3 rotational transitions of O2 in a broad sample of molecular clouds. I will discuss the HOP observations to date, focusing on the detection of O2 towards the H2 Peak 1 position near KL in Orion. This region has some interesting compact molecular sources that have emission in the same velocity range as found for the observed O2 lines and we explore a model based on warming grains and restoration of gas phase chemistry that can explain our observations. This region is also heavily impacted by the molecular outflow and resulting shocks which are manifest in the highly-structured emission from pure rotational and rotation-vibration transitions of H2. The detection of O2 at several positions in ρ Oph and the non-detection in the Orion Bar PDR suggest possibly contradictory explanations for the O2 abundance in various environments. I conclude with a discussion of some of the implications for oxygen chemistry in dense interstellar clouds.