Upper panel: evolution of the fractional abundance of SiO in the gas phase (left ordinate, full curves) and temperature of the neutral fluid (right ordinate, broken curves). From left to right: dust modelling scenarios M1, M2, and M3. The models shown in all three panels are for V_s = 20 km s^-1 with b = 1.0 (blue), V_s = 30 km s^-1 with b = 1.5 (green), and V_s = 40 km s^-1 with b = 2.0 (red). The fractional abundance of SiO in the gas phase is negligible for V_s = 20 km s^-1 with b = 1.0 in scenario M2. Lower panel: variation through the shock wave of the gas-phase fractional abundances of O₂ (left ordinate, full blue curves) and OH (left ordinate, full red curves), together with the temperature of the neutral fluid (right ordinate, black broken curves), for V_s = 30 km s^-1 and b = 1.5 and each of the dust models M1, M2 and M3 (from left to right).