A photon dominated region code comparison study^*

¹ Argelander-Institut für Astronomie (Founded by merging of the Sternwarte, Radiastronomisches Institut and Institut für Astrophysik und Extraterestrische Forschung.) , Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: roellig@ph1.uni-koeln.de
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ University of Kentucky, Department of Physics and Astronomy, Lexington, KY 40506, USA
⁴ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁵ California Institute of Technology, 1200 E. California Blvd, Pasadena CA 91125, USA
⁶ Onsala Space Observatory, 439 92 Onsala, Sweden
⁷ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Space Telescope Science Division of ESA, Space Telescope Science Institute, Baltimore, MD 21218, USA
⁹ Department of Physics, San Jose State University, 1 Washington Square, San Jose, CA 95192, USA
¹⁰ LUTH UMR 8102, CNRS and Observatoire de Paris, Place J. Janssen, 92195 Meudon Cedex, France
¹¹ LAEFF, Villafranca del Castillo, Apdo. 50727, 28080 Madrid, Spain
¹² SRON National Institute for Space Research, Postbus 800, 9700 AV Groningen, The Netherlands
¹³ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁴ School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Israel
¹⁵ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁶ Royal Observatory of Belgium, Av. Circulaire, 3 - Ringlaan 3, 1180 Brussel, Belgium
¹⁷ Astronomy Department, University of Maryland, College Park, MD 20742-2421, USA

Received: 27 June 2006
Accepted: 2 February 2007

Abstract

Aims.We present a comparison between independent computer codes, modeling the physics and chemistry of interstellar photon dominated regions (PDRs). Our goal was to understand the mutual differences in the PDR codes and their effects on the physical and chemical structure of the model clouds, and to converge the output of different codes to a common solution.

Methods. A number of benchmark models have been created, covering low and high gas densities  cm^-3 and far ultraviolet intensities χ = 10, 10⁵ in units of the Draine field (FUV: 6 <   < 13.6 eV). The benchmark models were computed in two ways: one set assuming constant temperatures, thus testing the consistency of the chemical network and photo-processes, and a second set determining the temperature self consistently by solving the thermal balance, thus testing the modeling of the heating and cooling mechanisms accounting for the detailed energy balance throughout the clouds.

Results.We investigated the impact of PDR geometry and agreed on the comparison of results from spherical and plane-parallel PDR models. We identified a number of key processes governing the chemical network which have been treated differently in the various codes such as the effect of PAHs on the electron density or the temperature dependence of the dissociation of CO by cosmic ray induced secondary photons, and defined a proper common treatment. We established a comprehensive set of reference models for ongoing and future PDR model bench-marking and were able to increase the agreement in model predictions for all benchmark models significantly. Nevertheless, the remaining spread in the computed observables such as the atomic fine-structure line intensities serves as a warning that there is still a considerable uncertainty when interpreting astronomical data with our models.