Two prototypical Galactic PDRs:The Orion bar and the Horsehead mane

Abstract

Photodissociation region (PDR) models are used to understand the evolution of the FUV illuminated matter both in our Galaxy and in external galaxies. To prepare for the unprecedented spatial and spectroscopic capabilities of ALMA and Herschel, two different kinds of progresses are currently taking place in the field. First, numerical models describing the chemistry of a molecular cloud are being benchmarked against each other to ensure that all models agree not only qualitatively but also quantitatively on at least simple cases (Röllig et al. 2007). Second, new or improved chemical rates are being calculated/measured by several theoretical and experimental groups. However, the difficulty of this last effort implies that only a few reactions (among the thousands used in chemical networks) can be thoroughly studied. New numerical tools are thus being developed for taking into account the impact of the uncertainties of the chemical rates on the chemical model predictions and their comparison with observed abundances (e.g. Wakelam et al. 2005, 2006). In view of the intrinsic complexity of building reliable chemical networks and models, there is an obvious need for well-defined observations that can serve as basic references. PDRs are particularly well suited to serve as references because they are the link between diffuse and dark clouds, thus enabling to probe a large variety of physical and chemical processes. We describe the merit of two prototypical PDRs: the Orion Bar and the Horsehead mane.