Observed abundances of molecular ions (colour vertical bars) are compared with theoretical values calculated with Astrochem (colour lines). The colours indicate HCO⁺ (black), N₂H⁺(blue), HOCO⁺ (red), SO⁺ (green), and HCS⁺ (magenta) abundances. PanelA) “STEP 1”: steady-state abundances in the cloud, X = N_species/N_H2, as a function of the gas density, n_H2 (cm^-3), for gas temperature T_kin = 10 K, and cosmic ray ionisation rate ζ = 3 × 10^-17,10^-16,3 × 10^-16 s^-1 (dotted, dashed, and solid lines). Observed abundances are overplotted at the assumed cloud density (n_H2= 10⁴ cm^-3, vertical dotted line). PanelB) “STEP 2”: evolution of steady-state abundances in the shock as a function of time (in years). The gas density and temperature are raised from cloud values (n_H2= 10⁴ cm^-3, T_kin = 10 K) to the observed values in the outflow cavity (n_H2= 10⁵ cm^-3, T_kin = 70 K). Observed abundances are overplotted at the shock age (t_shock ~ 2000 years, vertical dotted line).