Rotational diagram of ¹²CO (left) and ¹³CO (right) lines observed in the Orion Bar (top panel) and NGC 7023 (bottom panel) PDRs. For ¹²CO, the excitation temperature and total column density are computed in the range J _up = [15, 23] (Orion Bar), and J _up = [15, 19] (NGC 7023). For ¹³CO, the lines used were J _up = [5, 16] (Orion Bar), and J _up = [5, 10] (NGC 7023).