2009A&A...498..161V

Photon-dominated regions (PDRs) are expected to show a layered structure in molecular abundances and emerging line emission, which is sensitive to the physical structure of the region as well as the UV radiation illuminating it. We aim to study this layering in the Orion Bar, a prototypical nearby PDR with a favorable edge-on geometry. We present new maps of 2'x2' fields at 14''-23'' resolution toward the Orion Bar in the SO 8₈-9₉, H₂CO 5₁₅-4₁₄, ¹³CO 3-2, C₂H 4_9/2-3_7/2 and 4_7/2-3_5/2, C¹⁸O 2-1 and HCN 3-2 transitions. The data reveal a clear chemical stratification pattern. The C₂H emission peaks close to the ionization front, followed by H₂CO and SO, while C¹⁸O, HCN and ¹³CO peak deeper into the cloud. A simple PDR model reproduces the observed stratification, although the SO emission is predicted to peak much deeper into the cloud than observed while the H₂CO peak is predicted to peak closer to the ionization front than observed. In addition, the predicted SO abundance is higher than observed while the H₂CO abundance is lower than observed. The discrepancies between the models and observations indicate that more sophisticated models, including production of H₂CO through grain surface chemistry, are needed to quantitatively match the observations of this region.