2005A&A...435..885P

We present maps at high spatial and spectral resolution in emission lines of CCH, c-C₃H₂, C₄H, ¹²CO and C¹⁸O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H₂ ro-vibrational lines and in the PAH mid-infrared bands. C¹⁸O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C₃H₂ and C₄H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H₂ peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.