Herschel search for O 2 toward the Orion Bar

We report the results of a search for molecular oxygen (O-2) toward the Orion Bar, a prominent photodissociation region at the southern edge of the HII region created by the luminous Trapezium stars. We observed the spectral region around the frequency of the O-2 N-J = 3(3)-1(2) transition at 487 GHz and the 5(4)-3(4) transition at 774 GHz using the Heterodyne Instrument for the Far-Infrared on the Herschel Space Observatory. Neither line was detected, but the 3 sigma upper limits established here translate to a total line-of-sight O-2 column density <1.5 x 10(16) cm(-2) for an emitting region whose temperature is between 30 K and 250 K, or <1 x 10(16) cm(-2) if the O-2 emitting region is primarily at a temperature of less than or similar to 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 O-2 column density is less than 4 x 10(15) cm(-2), a value that is below, and possibly well below, model predictions for gas with a density of 10(4)-10(5) cm-(3) exposed to a far-ultraviolet flux 10(4) 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 AV of the Bar is less than required for O-2 to reach peak abundance; (3) the O-2 emission arises within dense clumps with a small beam filling factor; or (4) the face-on depth into the Bar where O-2 reaches its peak abundance, which is density dependent, corresponds to a sky position different from that sampled by our Herschel beams.