Critical number of cluster members where t_ed = τ_k for the Herbig model (top, lines) and the T Tauri model (bottom, lines). The smallest stable molecule is indicated by the filled colour. For the Herbig star model, clusters are stable outside the regions where they can possibly form. In the T Tauri disc model, circumcoronene and circumcircumcoronene can be stabilised where clusters can form in the midplane. Coronene clusters are unlikely to survive.