Carbon budget and carbon chemistry in Photon Dominated Regions

We present a study of small carbon chains and rings in Photon Dominated Regions (PDRs) performed at millimetre wavelengths. Our sample consists of the Horsehead nebula (B33), the ρ Oph L1688 cloud interface, and the cometary-shaped cloud IC63. Using the IRAM 30-m telescope, the SEST and the Effelsberg 100-m telescope at Effelsberg., we mapped the emission of C2H, c-C3H2 and C4H, and searched for heavy hydrocarbons such as c-C3H, l-C3H, l-C3H2, l-C4H2 and C6H. The large scale maps show that small hydrocarbons are present until the edge of all PDRs, which is surprising as they are expected to be easily destroyed by UV radiation. Their spatial distribution reasonably agrees with the aromatic emission mapped in mid-IR wavelength bands. C2H and c-C3H2 correlate remarkably well, a trend already reported in the diffuse ISM (Lucas and Liszt 2000). Their abundances relative to H2 are relatively high and comparable to the ones derived in dark clouds such as L134N or TMC-1, known as efficient carbon factories. The heavier species are however only detected in the Horsehead nebula at a position coincident with the aromatic emission peak around 7 µm. In particular, we report the first detection of C6H in a PDR. We have run steady-state PDR models using several gas-phase chemical networks (UMIST95 and the New Standard Model) and conclude that both networks fail in reproducing the high abundances of some of these hydrocarbons by an order of magnitude. The high abundance of hydrocarbons in the PDR may suggest that the photo-erosion of UV-irradiated large carbonaceous compounds could efficiently feed the ISM with small carbon clusters or molecules. This new production mechanism of carbon chains and rings could overcome their destruction by the UV radiation field. Dedicated theoretical and laboratory measurements are required in order to understand and implement these additional chemical routes.