Are PAHs precursors of small hydrocarbons in Photo–Dissociation Regions? The Horsehead case

We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, CO and CO of the edge of the Horsehead nebula obtained with the 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. C O 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 amount.