Column densities of HCN and HNC and their column density ratio as a function of N(HCO⁺), with predictions from isobaric PDR models overlaid. Filled and unfilled black markers indicate foreground components and molecular envelope components observed, respectively. The arrows refer to lower limits of N(X), and the markers are as same as Fig. 20. Each color represents different radiation field strengths and cosmic-ray ionization rates; red: G₀ = 1 and ζ = 5 × 10⁻¹⁷ s⁻¹ green: G₀ = 100 and ζ = 5 × 10⁻¹⁷ s⁻¹ blue: G₀ = 1 and ζ = 1 × 10⁻¹⁶ s⁻¹ orange: G₀ = 100 and ζ = 1 × 10⁻¹⁶ s⁻¹ and purple: G₀ = 100 and ζ = 1 × 10⁻¹⁵ s⁻¹ Initial pressures of 1000, 3000, 5000, 10 000 K cm⁻³ are indicated with dotted-, dashed-, dashed-dotted-, and solid-lines, respectively. A_v =1, 5, and 10 mag are marked with colored triangles, squares, and circles, respectively.