Effect of dust extinction optical depth on the 11.2/3.3 μm ratio. The dust extinction cross-sections per H-atom have been taken from Draine & Li (2001); the model with RV = 5.5). For comparison, the maximum H-column density along the line of sight in the northern PDR of NGC 7023 is measured to be 2.2 × 1021 atoms cm-2 (Fuente et al. 1998).
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