To prepare for the unprecedented spatial and spectral resolution provided by ALMA and Herschel/HIFI, chemical models are being benchmarked against each other. It is obvious that chemical models also need well-constrained observations that can serve as references. Photo-dissociation regions (PDRs) are particularly well suited to serve as references because they make the link between diffuse and molecular clouds, thus enabling astronomers to probe a large variety of physical and chemical processes. At a distance of 400 pc (1 corresponding to 0.002 pc), the Horsehead PDR is very close to the prototypical kind of source (i.e. 1D, edge-on) needed to serve as a reference to models.
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