ALMA ¹²CO observations of the shell for ¹²CO(1−0), ¹²CO(2−1), and ¹²CO(3−2) (black lines, panels 1–3, respectively). The data are extracted from annuli within ±3′′ from the projected radius of the shell in each velocity bin in the images smoothed to 4′′ resolution. For comparison, the black dotted lines show the emission integrated over the entire image for each velocity bin instead (i.e. also containing emission from the CSE). The red lines show the best-fit model of the shell only. The right panel shows the χ² map of the radiative transfer models of the shell with shell-mass vs. shell temperature. The colour scale gives the χ² value, the white cross indicates the best-fit model. The white contours give the 1σ and 2σ levels (the 3σ contours fall outside of the plotted range).