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 the Heterodyne Instrument for the Far Infrared 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 × 1016 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) × 10−6. 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 and the dust temperature is 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T 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 180 K. The inferred O2 column density 5 × 1018 cm−2 can be produced in Peak A, having N(H2) 4 × 1024 cm−2. An alternative mechanism is a low-velocity (10–15 km s−1) C-shock, which can produce N(O2) up to 1017 cm−2.