Prediction of the abundances relative to H₂ of C⁺, C, CO, CH, CCH  and c-C₃H₂  from the PDR Meudon models with various hydrogen densities and pressures. Models with A_V = 5   mag are plotted with open squares, A_V = 3   mag with black squares and A_V = 1   mag with crosses. Constant pressure models are plotted using the highest density point as abscissa. Models with elevated cosmic ray ionization fluxes are drawn in red, using larger symbols. The gray bars indicate the observed abundances of CH (Sheffer et al. 2008), CCH (Gerin et al. 2010) and c-C₃H₂(this work, using the scaling with CCH).