Multi-line detection of O2 toward rho Oph A
  Models of pure gas-phase chemistry in well-shielded regions of molecular
clouds predict relatively high levels of molecular oxygen, O2, and water, H2O.
Contrary to expectation, the space missions SWAS and Odin found only very small
amounts of water vapour and essentially no O2 in the dense star-forming
interstellar medium. Only toward rho Oph A did Odin detect a weak line of O2 at
119 GHz in a beam size of 10 arcmin. A larger telescope aperture such as that
of the Herschel Space Observatory is required to resolve the O2 emission and to
pinpoint its origin. We use the Heterodyne Instrument for the Far Infrared
aboard Herschel to obtain high resolution O2 spectra toward selected positions
in rho Oph A. These data are analysed using standard techniques for O2
excitation and compared to recent PDR-like chemical cloud models. The 487.2GHz
line was clearly detected toward all three observed positions in rho Oph A. In
addition, an oversampled map of the 773.8GHz transition revealed the detection
of the line in only half of the observed area. Based on their ratios, the
temperature of the O2 emitting gas appears to vary quite substantially, with
warm gas (> 50 K) adjacent to a much colder region, where temperatures are
below 30 K. The exploited models predict O2 column densities to be sensitive to
the prevailing dust temperatures, but rather insensitive to the temperatures of
the gas. In agreement with these model, the observationally determined O2
column densities seem not to depend strongly on the derived gas temperatures,
but fall into the range N(O2) = (3 to >6)e15/cm^2. Beam averaged O2 abundances
are about 5e-8 relative to H2. Combining the HIFI data with earlier Odin
observations yields a source size at 119 GHz of about 4 - 5 arcmin,
encompassing the entire rho Oph A core.
