OH emission from warm and dense gas in the Orion Bar PDR

As part of a far-infrared (FIR) spectral scan with Herschel/PACS, we present the first detection of the hydroxyl radical (OH) towards the Orion Bar photodissociation region (PDR). Five OH (X (2)Pi; nu = 0) rotational.-doublets involving energy levels out to E-u/k similar to 511 K have been detected (at similar to 65, similar to 79, similar to 84, similar to 119 and similar to 163 mu m). The total intensity of the OH lines is Sigma I(OH) similar or equal to 5x10(-4) erg s(-1) cm(-2) sr(-1). The observed emission of rotationally excited OH lines is extended and correlates well with the high-J CO and CH+ J = 3-2 line emission (but apparently not with water vapour), pointing towards a common origin. Nonlocal, non-LTE radiative transfer models including excitation by the ambient FIR radiation field suggest that OH arises in a small filling factor component of warm (T-k similar or equal to 160-220 K) and dense (n(H) similar or equal to 106-7 cm(-3)) gas with source-averaged OH column densities of greater than or similar to 10(15) cm(-2). High density and temperature photochemical models predict such enhanced OH columns at low depths (AV less than or similar to 1) and small spatial scales (similar to 10(15) cm), where OH formation is driven by gas-phase endothermic reactions of atomic oxygen with molecular hydrogen. We interpret the extended OH emission as coming from unresolved structures exposed to far-ultraviolet (FUV) radiation near the Bar edge (photoevaporating clumps or filaments) and not from the lower density interclump medium. Photodissociation leads to OH/H2O abundance ratios (>1) much higher than those expected in equally warm regions without enhanced FUV radiation fields.