Relative abundances of HCS⁺ and CS as a function of A_V. The boxes are the observations with error bars, and the lines are the model results with  [S]  = 10^-5 (red),  [S]  = 10^-6 (green) and  [S]  = 7.2 × 10^-8 (blue), all using the high column-dependent ζ from Fig. 2. The solid lines use a rate for S + H⁺ → S⁺ + H of 1.3 × 10^-9 cm³ s^-1 (Prasad & Huntress 1980) and the dashed lines use a rate of 1 × 10^-14 cm³ s^-1.