Volume 530, June 2011
|Number of page(s)||6|
|Published online||25 May 2011|
Letter to the Editor
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.