Model DR_P at t = 1200 yr under an inclination of 30°. Top: map of H₂ column density for j_mix = 1.0. The location of the apertures A1-A4 used to extract the mass–velocity relationships are drawn with black circles. Bottom: mass–velocity relationship averaged over the circular apertures A1 to A4. The best exponential fits are drawn in dashed red. The value of v₀ is given for each value of jet–ambient material mixing ratio j_mix = 0, 0.3, 0.6, 1.0.