Predicted HCNH⁺/HCN ratio for the CM (green solid line) and for the WM (dashed green line). The grey shaded areas represent the range of observed HCNH⁺/HCN ratios for the CM (filled area), and for the WM (striped area). The purple vertical lines indicate the times at which the abundances of HCNH⁺, HCN, and HCO⁺ are reproduced by the CM (t_CM = 4 × 10⁵ yr, solid line), and by the WM (t_WM = 5 × 10⁴ yr, dashed line).