Radial profiles and correlation between polarisation fraction and surface brightness for alternative models with d = 231 pc, as indicated in the last frame. The shaded regions correspond to the inter-quartile intervals for the models L₃ (green) and M₅ (blue). Frames a and b: additional curves for a S/N > 10, T_d < 13 K sub-sample of the default case (dashed black lines). In frame b, these are on top of each other. Frame c: curves for the default and H cases also overlap.