DCN/HCN abundance ratio in logarithmic scale as a function of time (Phase II) as predicted by our model with a pre-shock density of n(H₂) = 10⁵ cm^-3 and a shock velocity v_s = 40 km s^-1. The thin red line displays the neutral gas temperature profile and the dotted red line the temperature of the ions. Black thick solid line depicts the model with the standard value of the cosmic ionization rate while the thick dashed-line shows the model with ζ = 1.3 × 10^-16 s^-1. The gray area shows the range of observed values.