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 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 _sun 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 sime5 × 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.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.