Comparison between observations (left panel) and a toy model (right panel) which reproduces the line-of-sight velocity pattern of ammonia. Left: same as the bottom panel of Fig. 1, but with the axes labelled in astronomical units and offsets counted from the star position. A grid is drawn at steps of 200 au for a direct comparison with the right panel and the quadrants are labelled. The third quadrant shows a velocity gradient that is much smoother than in the first quadrant due to ammonia linewidths being twice as broad (Fig. 4). Right: similar to the left panel, but with the colour map obtained from the toy model of a razor thin disk with the same geometry of circumstellar gas around HW2 and convolved with the same instrumental beam. Model parameters are written in the top-left, with the velocity vectors sketched in the fourth quadrant.