| Description |
Oxygen is the third most abundant element in the Universe and as such it plays a decisive role in the chemistry and physics of molecular clouds. Chemical models have long predicted that some of the simplest molecules would be the primary reservoirs of oxygen in space: CO, H2O, and O2, and under these conditions would be major cloud coolants influencing the evolution of the cloud and the process of star formation. Molecular oxygen has remained elusive, with results from SWAS and Odin missions indicating abundances approximately two orders of magnitude below those predicted by standard gas phase chemistry models. The favored explanation is that oxygen atoms are tied up as water ice on grain surfaces in the cold, extended regions of clouds probed with the relatively large beams of previous space observatories. We propose to use Herschel HIFI to carry out a survey of regions in which the O2 abundance is predicted to be large due to reestablishment of standard gas phase chemistry. We will observe a selected group of sources including regions of heated gas surrounding embedded young stars, photon dominated regions, X-ray dominated regions, and shock-heated regions, in the 487 and 774 GHz lines of O2, and a subset of these in the 1121 GHz line. We expect that these sources will be of small angular size, and can be observed using beam switching in mini line survey mode, to enable sideband deconvolution and minimize interference from confusing lines of other species. The greatly improved sensitivity of HIFI receivers and the far smaller Herschel beam relative to SWAS and Odin allow us to probe these regions to levels below those predicted by sophisticated astrochemical models. By measuring the O2 abundance, we can verify important aspects of models of these regions, put constraints on key gas-grain interaction parameters, and probe critical chemistry and physics in regions that are tracers of recent and prospective star formation. |
