a) Density structure n of the champagne flow as σ Ori moves toward the cloud (Sect. 3.3). The different curves represent the density structure after time t: 0 Myr, 1 Myr, 2 Myr, and 3 Myr. The dotted line shows the evolution of the density at the IF, n₀. The gray dashed box shows the outline of the region of panel c). b) Velocity structure of the flow. To transform this to the frame between star and cloud, one has to add the relative velocity between the star and cloud, v_⋆ = 10 km s^-1. c) Density profile at t = 3 Myr, overplotted with the observed density profile from the Hα emission measure, assuming a constant scale size of the emitting gas along the line of sight of l = 9 pc (see text).