2012ApJ...752...26M

We report the results of a search for molecular oxygen (O₂) toward the Orion Bar, a prominent photodissociation region at the southern edge of the H II region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O₂N_J= 3₃-1₂transition at 487 GHz and the 5₄-3₄ transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3σ upper limits established here translate to a total line-of-sight O₂ column density <1.5x10¹⁶/cm2 for an emitting region whose temperature is between 30 K and 250 K, or <1x10¹⁶/cm2 if the O₂ emitting region is primarily at a temperature of ≲ 100 K. Because the Orion Bar is oriented nearly edge-on relative to our line of sight, the observed column density is enhanced by a factor estimated to be between 4 and 20 relative to the face-on value. Our upper limits imply that the face-on O₂ column density is less than 4x10¹⁵/cm2, a value that is below, and possibly well below, model predictions for gas with a density of 10⁴-10⁵/cm3 exposed to a far-ultraviolet flux 10⁴ times the local value, conditions inferred from previous observations of the Orion Bar. The discrepancy might be resolved if (1) the adsorption energy of O atoms to ice is greater than 800 K; (2) the total face-on A_V of the Bar is less than required for O₂ to reach peak abundance; (3) the O₂ emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O₂ reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.