Other
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21720206g&link_type=abstract
American Astronomical Society, AAS Meeting #217, #202.06; Bulletin of the American Astronomical Society, Vol. 43, 2011
Other
Scientific paper
Oxygen is the third most abundant element in the cosmos. In the gas phase, oxygen can be ionized, atomic, or in molecular, and it is also incorporated into grains. Gas-phase chemistry models predict molecular oxygen (O2) to be almost as abundant as carbon monoxide (CO). A number of searches for molecular oxygen have been carried out, including ground-based searches for the isotopologue 16O18O and searches for O2 in redshifted galaxies. Searches for Galactic O2 carried out with the SWAS and Odin spacecraft have yielded upper limits on the abundance of molecular oxygen typically 1 to 2 orders of magnitude below those predicted by gas-phase models. There has been a fairly clear detection of O2 in one source, again indicating a low abundance. 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. Available gas-phase oxygen is largely incorporated into CO, leaving little for gas-phase O2. Other models involve circulation of material between UV-irradiated and well-shielded regions.
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. The sensitivity and angular resolution of HIFI is a dramatic improvement over anything previously available at these frequencies. These data should, whether yielding detections or significantly improved upper limits, provide critical information about interstellar chemistry and the structure of these varied molecular regions. We will discuss the HOP observations to date including exceptionally low upper limits to the abundance of O2 as well as some provocative, tentative positive results.
Goldsmith Paul
Herschel Oxygen Project HOP Team
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