Cosmic Rays in the Orion Bar

Abstract

The detailed structure of HII regions and the surrounding photodissociation regions (more precisely, the extent and chemical composition of the ionized, neutral, and molecular layers) poses strong constraints on the intensity of the magnetic field and the presence of cosmic rays. Conversely, an accurate knowledge of these physical parameters is necessary in order to understand the observable properties of nearby regions, as well as those of unresolved, distant star-forming galaxies. Due to its close distance to the Earth and its nearly edge-on geometry, the Orion Bar provides an excellent testbed for detailed photoionization models. We will show that, although the observed structure of the ionized HII region can be reproduced with remarkable accuracy, the agreement in the atomic layer is only qualitative, and the predicted abundances of several molecular species are off by several orders of magnitude. Consistent with previous work, our results hint that the density of cosmic rays is high (well above the Galactic background, though not as large as implied by equipartition arguments), but their role on the molecular chemistry is still unclear. In particular, we consider that cosmic rays alone cannot probably solve all the present discrepancies between models and observations, and a more detailed treatment of the three-dimensional structure of the region is clearly required.