Observed atomic line ratios compared to C-type (red dashed lines) and J-type (blue lines) shock model predictions from Flower & Pineau des Forêts (2015) for shock velocities (V_shock) between 10 and 40 km s^-1. The circles, diamonds, triangles, and squares correspond to pre-shock densities (n) of 10³ cm^-3, 10⁴ cm^-3, 10⁵ cm^-3, and 10⁶ cm^-3, respectively. The range of the observed atomic ratios is represented by the shaded region.