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

Abstract

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 2Π; 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 (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, 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 (Tk 160-220 K) and dense (nH 106-7 cm-3) gas with source-averaged OH column densities of 1015 cm-2. High density and temperature photochemical models predict such enhanced OH columns at low depths (AV 1) and small spatial scales (∼1015 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. © 2011 ESO.