Hydrostatic photoionization models of the Orion Bar
  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.
