Integrated intensities of the SiO lines relative to SiO(5−4). The horizontal lines show the values of the SiO(8–7) to (5−4) ratio, whether it is the one in the blue or red lobe of the outflow. The vertical line marks the position of the (8–7) transition on the X-axis. The colour curves provide the results for the fraction of our C-type shock models grid relevant to our study. For all these models, the pre-shock density is n_H = 10⁶ cm^-3, and the magnetic field parameter is b = 1. Only the shock velocity ν_s varies, and is indicated for each model on the corresponding line. The blue- and red-velocity range are defined in Fig. 6.