Spectral-index-radial profiles calculated following Equation 2 between 1.3 mm and 3.0 mm (left panel), between 1.3 mm and 8.9 mm (middle panel), and between 3.0 mm and 8.9 mm (right panel). The shaded area correspond to the error, computed from the 1σ standard deviation of the intensity at each radius. The Gaussian inset indicates the geometric average of the beam size, knowing that all the observations have been convolved to a common resolution of 0.12″ × 0.12″. The green arrow in each panel represents the maximum shift possibly introduced by flux calibration systematic errors, which applies equally at each radius. Grey and black vertical-dashed lines correspond respectively to the cavity radius R_cav and peak intensity radius R_peak at 1.3 mm. Beyond 56 au, negative values in the 3.0 mm data make the calculations fail.