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

摘要

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 ?∑?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, 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.