a) SED in νL_ν for each object normalized at λ = 2.2 μm, plotted against normalized half-light radius R_1/2/R_in, for 2.2, 8.5, and 13 μm (purple, green, and red, respectively). b) The same data but with the grid in dashed lines for temperature/density gradient models with radial surface density indices of α =  −0.4, −0.8, −1.2, −1.6 for the case of T ∝ r^-0.36, plotted for the three wavelengths 2.2, 8.5, and 13 μm (purple, green, and red squares). This simple power-law model does not explain the observed SED and size at the same time. c) The same data but with the grid in dashed lines for the power-law (density  ∝ r^{−0,−0.2,−0.4,−0.6}, T ∝ r^-0.36) + 1400 K thin-ring at R_in, which roughly explain the observed SED and size. The two reddened sources IRAS13349 and H0557-385 could be interpreted in terms of the same model with a slightly larger filling factor for the power-law component.