Ratio between the observed and the modeled surface brightnesses at 1.2 mm (top) and 2 mm (bottom) as a function of geometric mean radius of the ellipses in Fig. 6, using the temperature and density profiles from Keto et al. (2015) and an opacity value of κ_{250 μm} = 0.2 ± 0.1 cm² g^-1 and β = 2.3 ± 0.4. The shadowed red region represents the uncertainties on the fluxes, the opacity, and the spectral index. The shaded blue region indicates the zone where the observed surface brightness is below 3σ.