Herschel Measurements of Molecular Oxygen in Orion
  We report observations of three rotational transitions of molecular oxygen
(O2) in emission from the H2 Peak 1 position of vibrationally excited molecular
hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using
HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km
s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) =
6.5\times1016 cm-2, and assuming that the source has an equal beam filling
factor for all transitions (beam widths 44, 28, and 19"), the relative line
intensities imply a kinetic temperature between 65 K and 120 K. The fractional
abundance of O2 relative to H2 is 0.3 - 7.3\times10-6. The unusual velocity
suggests an association with a ~ 5" diameter source, denoted Peak A, the
Western Clump, or MF4. The mass of this source is ~ 10 M\odot and the dust
temperature is \geq 150 K. Our preferred explanation of the enhanced O2
abundance is that dust grains in this region are sufficiently warm (T \geq 100
K) to desorb water ice and thus keep a significant fraction of elemental oxygen
in the gas phase, with a significant fraction as O2. For this small source, the
line ratios require a temperature \geq 180 K. The inferred O2 column density
\simeq 5\times1018 cm-2 can be produced in Peak A, having N(H2) \simeq
4\times1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1)
C-shock, which can produce N(O2) up to 1017 cm-2.
