Variation of excitation temperature (blue solid lines) and optical depth (red dotted lines) as a function of H₂ -density for the 1 → 0 (left panels) and 2 → 1 (right panels) CF⁺ rotational transitions at kinetic temperatures of 20 K (upper panels), 50 K (middle panels), and 100 K (lower panels), and a CF⁺ column density of N(CF⁺) = 1 × 10¹² cm⁻². In order to better display the large amplitude of its variation, the excitation temperature is represented on this figure as .