Hydrostatic photoionization models of the Orion Bar

Abstract

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 HII regions and the surrounding photon-dominated regions. In the present 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 HII 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.