Disc models of narrow planetesimal belts centred at 40 au with different assumptions for the stellar wind (SW) strength (from left to right Ṁ_⋆ = 0,50,300 Ṁ_⊙). a)−c): SEDs. Crosses show photometric data and squares star-subtracted photometric data. The dashed lines depict the stellar photosphere, the dash-dotted lines the disc emission, and the solid lines the star+disc emission. The were obtained using the N = 20 photometric points in the plotted range. d)−f): normal optical thickness as a function of grain size at different distances from the star. The solid curves (r = 39 au) show the particle distribution within the parent belt. Shaded regions depict the contribution of grains on unbound orbits at r = 39 au. g)−i): normal optical thickness as a function of distance from the star. Shaded regions indicate the contributions from different size ranges.