Comparison between observed value (horizontal dotted line) and theoretical values for the Flower & Pineau des Forêts (2010) shock models (connected symbols) of the [O i]145 μm  surface brightness (left panel) and the cooling ratio L([O i]145+63)/L(CO) (right panel), as a function of shock speed. Solid lines refer to J-shocks and dashed lines to C-shocks; squares refer to pre-shock density n = 2 × 10⁴ cm^-3  and triangles to n = 2 × 10⁵ cm^-3.