The H₂ brightness in the v = 1–0 S(1) 2.12 μm line against the v = 2–1 S(1)/v = 1–0 S(1) ratio for two positions in the DG Tau blue lobe (filled triangles with error bars) and for the grid of planar C-shock models with b = 0.5 (color curves) calculated by Kristensen et al. (2008). Each curve corresponds to a different value of the preshock hydrogen nucleus density, increasing from bottom to top: n_H= 10⁴, 10⁵, 10⁶ and 10⁷ cm^-3. The shock speed V_s increases to the right and some values are marked along the curves to guide the eye.