Upper panels: the degree of ionization (traced by HCO⁺/¹³CO) versus the gas-to-dust ratio (¹³CO/F_1.3mm), the difference in the mid-infrared spectral slope between 6, 10, and 25 μm (Δn), and the strength of the 10 μm silicate emission feature. Lower panels: similar plots for the photodissociation effect (traced by CN/HCN). See Fig. 5 for an explanation of the symbols. (Color plots are available online.)