Depletion Cores - the O2 hideout?

Molecular oxygen has proven to be the most elusive molecule in the interstellar medium. Despite the fact that it in theory forms easily in both warm and cold dense gas, extensive searches with SWAS, Odin and Herschel have only resulted in detections in a handful of sources. In addition, upper limits in various astronomical environments are at levels of 1000 times less abundant than predicted by chemical models. This situation requires either for atomic carbon to be abundant enough to suppress the O2 by CO formation, or for atomic oxygen to accrete onto grains and remain bound there. However, the binding energies of atoms to grains are highly uncertain and high abundances of OI in depleted gas have both been directly observed and inferred from observations of other molecules. A possible explanation is that OI is bound to grains by fixing (get hydrogenated to form ices) rather than sticking (van der Waals bonding to the surface potential), which will become less efficient in high density gas. We will present a stochastic gas-grain model including the kinetics of OI fixing - demonstrating the possibility of elevated O2 abundances at times when CO is significantly depleted - as well as results from searches for O2 emission in a small sample of starless depletion cores using Herschel HIFI