Journal Article

Hydrostatic photoionization models of the Orion Bar

Y. Ascasibar, A. C. Obreja and A. I. Díaz

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 2, pages 1546-1555
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI:
Hydrostatic photoionization models of the Orion Bar

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Due to its proximity to the Earth and its nearly edge-on geometry, the Orion Bar provides an excellent testbed for detailed models of the structure of H ii regions and the surrounding photon-dominated regions. In this study, a self-consistent model of the structure of the Orion Nebula in the vicinity of the Bar is built under the assumption of approximate ionization, thermal and hydrostatic equilibrium. It is found that a fairly simple geometry is able to describe the surface brightness profiles of the emission lines tracing the ionized H ii region with a remarkable accuracy, independent of the prescription adopted to set the magnetic field or the population of cosmic rays. Although we consider different scenarios for these non-thermal components, none of the models is able to provide a fully satisfactory match to the observational data for the atomic layer, and the predicted column densities of several molecular species are always well above the measured abundances. Contrary to previous studies, we conclude that a more elaborate model is required in order to match all the available data.

Keywords: H ii regions; ISM: individual objects: Orion Bar; ISM: individual objects: Orion Nebula; ISM: individual objects: M42; ISM: individual objects: NGC 1976; photodissociation region (PDR)

Journal Article.  7642 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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