2011A&A...530L..16G

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 (X²Π; ν=0) rotational Λ-doublets involving energy levels out to E_u/k∼511K have been detected (at ∼65, ∼79, ∼84, ∼119 and ∼163µm). The total intensity of the OH lines is ∑I(OH)≃5x10^–4 erg/s/cm²/sr. 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≃160-220K) and dense (n_H≃10^6–7cm^–3) gas with source-averaged OH column densities of >10¹⁵cm^–2. High density and temperature photochemical models predict such enhanced OH columns at low depths (A_V≲1) and small spatial scales (∼10¹⁵cm), 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.