¹³CO (6–5) in the Orion Bar: A critical observational test for PDR models

Abstract

The Orion Bar is a limb-brightened edge of the cavity carved out in the Orion molecular cloud by the expanding HII region surrounding the Trapezium cluster. The Bar is one of the best studied Photon-Dominated Regions (PDRs; see Genzel in this volume for a review). Tauber et al. (1995) found a tight correlation between the distributions of the neutral atomic carbon and ¹³CO (2−1) emission toward the Bar, which is difficult to explain in terms of classical PDR models. A possible explanation for this correlation was given by the geometric model of Hogerheijde et al. (1995) in which the PDR geometry changes from face-on to edge-on and back with increasing distance from the ionizing stars. The observed maximum of the molecular emission toward the Bar thus corresponds to a molecular column density peak and there are no significant variations in the excitation conditions across the Bar. In the present paper we investigate the uniformity of the excitation conditions across the Bar through observations of the ¹³CO (6−5) and ¹²CO (6−5) emission. We argue that the ¹³CO (6−5) emission provides a critical test for models of this region and PDRs in general.

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