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 Π; ν = 0) rotational Λ-doublets involving energy levels out to Eu/k ∼ 511 K have been detected (at ∼65, ∼79, ∼84, ∼119 and ∼163 μm). The total intensity of the OH lines is � I(OH) � 5 × 10 −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, nonLTE radiative transfer models including excitation by the ambient FIR radiation field suggest that OH arises in a small filling factor component of warm (Tk � 160–220 K) and dense (nH � 10 6−7 cm −3 ) gas with source-averaged OH column densities of 10 15 cm −2 . High density and temperature photochemical models predict such enhanced OH columns at low depths (AV 1) and small spatial scales (∼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.