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Context. Outflow cavities in envelopes of young stellar objects (YSOs) have been pr edicted to allow far-UV (FUV) photons to escape far from the central source, with significant observable e ffects, especially if the protostar is a forming high-mass star suspected of emitting a copious amount of FUV radiation. Indirect evidence of this picture has been provided by models and unresolved single-dish observations, but direct high-resolution data are necessary for con firmation. Previous chemical modeling has suggested that CS and HCN are good probes of the local FUV field, so make good target specie s. Aims. We directly probe the physical conditions of the material in the outflow walls to te st this prediction. Methods. Interferometric observations of the CS(7-6) and HCN(4-3) rotational lines in the high-mass star-forming region AFGL 2591 are carried out in the compact and extended configuration of the SubMillim eter Array (SMA). The velocity structure was analyzed, and integrated maps compared to K-band near-IR observations. A chemical model predicts abundances of CS and HCN for a gas under protostellar X-ray and FUV irradiation, and was used in conjunction with the data to distinguish between physical scenarios. Results. CS and HCN emission was found in spatial coincidence in extended sources displaced up to 7'' from the position of the young star. Their line widths are small, excluding major shocks. Chemical model calculations predict an enhanced abundance of the two molecules in warm, dense, and FUV irradiated gas. Hot dust observed between the molecular emission and the outflow accounts for the necessary attenuation to prevent photodissociation of the molecule s. Conclusions. The SMA data suggest that the outflow walls are heated and chemically altere d by the FUV emission of the central high-mass object, providing the best direct evidence yet of large-sca le direct irradiation of outflow walls.

Evidence of warm and dense material along the outflow of a high-mass YSO