C¹⁸O J = 2−1 ALMA simulations for the representative Class I model. The first five panels from top left to the bottom right show integrated intensity maps for the post-burst phase at 10, 1000, 3000, 10⁴ and 10⁵ yr years after the end of the burst (see also Fig. 3). The target is seen face-on (looking down the outflow cavity, inclination =0°). The white ellipse in the top left plot shows the synthetic beam (1.81″ × 1.65″). The linear scale for the intensity is shown in the bottom centre plot. The white and yellow contour lines show 50% and 20% of the peak intensity level for each map. The plot at the bottom right shows the azimuthally averaged intensity profiles for the different times. The dots in this plot mark the full width half maximum of the profile (R_50%).