Results obtained with the Paris-Durham shock model. Each panel corresponds to a different pre-shock density: n_H = 100 (left) and 500 cm⁻³ (right). The three coloured rectangles (blue, red, and green) show the observational results: the (2-1)/(1-0), (3-2)/(1-0), and (4-3)/(1-0) ratios, respectively (from Table 2). Each point corresponds to a set of {n_H, G₀, and v_s} values; v_s values are indicated in the X-axis. The model results are shown with symbols: filled or empty for G₀ = 0 or 1, respectively.