Volume 530, June 2011
|Number of page(s)||6|
|Published online||25 May 2011|
Letter to the Editor
OH emission from warm and dense gas in the Orion Bar PDR⋆,⋆⋆
Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
2 Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
3 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
4 Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
5 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
6 LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, France
7 Observatoire de Paris, LUTH and Université Denis Diderot, Place J. Janssen, 92190 Meudon, France
8 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
9 I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
Received: 29 March 2011
Accepted: 11 May 2011
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.
Key words: astrochemistry / infrared: ISM / ISM: abundances / ISM: molecules
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendix B is available in electronic form at http://www.aanda.org
© ESO, 2011
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