¹³CO J = 6–5 PV diagram for different model setups for a spiral structure, with the x-axis going from −6 to 6′′and the y-axis from −20 to 20 km s^-1: panel 1: radial outflowing wind with density-enhanced shells, for a shell-intershell contrast of a factor 10, panel 2: basic model for the spiral wind structure, with parameters a = 1.45, p = 2, b = 0, c = 8 × 10¹² cm (or 16 R_⋆), and w = 50 at a PA along the orbital plane, panels 3–8: same as basic model but with one parameter changed, as indicated at the top of each panel. For each panel, the colour wedge extends from zero to highest intensity (in Jy/beam), with redder colours indicating a higher intensity.