Photon-Dominated Regions (or PhotoDissociation Regions, PDRs) are molecular clouds exposed to intense radiation where UV photons dominate much of the chemistry and heating. Since a large fraction of the interstellar medium consists of PDRs, a good understanding of their physical and chemical structure is essential. In the last twenty years, considerable effort has gone into constructing PDR models. Unlike models of the chemical evolution of shielded dense clouds, PDR models require a spatial treatment of the chemistry and the heating and cooling of the gas. The ultraviolet radiative transfer needs to be solved with depth into the cloud, in particular to model the transition from H to H2, and C+ to CO, including dust attenuation and self-shielding. At least 10 different groups have developed independent codes on this topic. While there is a general agreement among modelers about the qualitative trends, there are large quantitative differences in the results for key species and their lines. With upcoming new far-infrared facilities (SIRTF, SOFIA, Herschel, ALMA), it is essential that these differences are understood and – where necessary – corrected.
The aim of the workshop is to test and benchmark the various PDR codes for a variety of interstellar regions. Some of the questions to be addressed at the workshop are:
- Can we understand the differences found in various codes for the chemistry and thermal balance for a set of simple test problems?
- Can we agree on a standard set of species and reactions which are important for PDR modelling?
- Which code-specific parameters should be standardised? What key physical and chemical aspects still need improvements?
- How important is the geometry of our models?
- What are the key observational questions?
The meeting will run from Monday April 5 to Thursday April 8 2004. Formal presentations will be limited, with most of the time spent on round-table discussions and splinter meetings.