Model results for the time-evolution of the abundance of PAHs (C₆₆H_n), cages (C, C, CC, C, C), and C₂. These are normalized to the abundance of C₆₆H₂₀ at t = 0. The model calculations are made for distances of 5, 10, 15, 20, and 25′′ from the star HD 200775 in NGC 7023. Note that the dissociation of C₂ is not treated in our model, hence the C₂ abundances reported are only provided as an indication of how much C₂ is formed from the dissociation of cages. Similarly, the dissociation of the C is not included in the model and therefore its abundance is provided as an indication of how much C₆₀ is being destroyed. The grey shadowed regions indicate the uncertainty range implied by uncertainties of a factor of 10 on the rates of C₂ loss (see Sect. 4.3 for details). The approximate age of NGC 7023 is given, with orange shadowed regions representing the uncertainty on this value (see Sect. 3 for details).