Observed CN/HCN column density ratio compared with shock models with UV for a preshock density of 10⁴ cm⁻³ (Lehmann et al. 2020). The column densities are shown as a function of shock velocity for the precursor and post-shock regions (red circles) and the post-shock region alone (green squares). Column densities ratios obtained for Class 0 and Class I protostars are shown as dark gray and light gray rectangles, respectively. The horizontal lines show median CN/HCN ratios, and the vertical lines their standard deviations for Class 0 (red) and Class I (blue) sources.