2012A&A...541A..73L

Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O₂, and water, H₂O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O₂ in the dense star-forming interstellar medium. Only toward ρ Oph A did Odin detect a very weak line of O₂ at 119 GHz in a beam of size 10-arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O₂ emission and pinpoint its origin. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O₂ spectra toward selected positions in the ρ Oph A core. These data are analysed using standard techniques for O₂ excitation and compared to recent PDR-like chemical cloud models. The N_J=3₃-1₂ line at 487.2GHz is clearly detected toward all three observed positions in the ρ Oph A core. In addition, an oversampled map of the 5₄-3₄ transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O₂ emitting gas appears to vary quite substantially, with warm gas (>50K) being adjacent to a much colder region, of temperatures lower than 30K. The exploited models predict that the O₂ column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O₂ column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O₂)=3 to >6x10¹⁵cm^–2. Beam-averaged O₂ abundances are about 5x10^–8 relative to H₂. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire ρ Oph A core. We speculate that one of the reasons for the generally very low detection rate of O₂ is the short period of time during which O₂ molecules are reasonably abundant in molecular clouds.