χ² obtained by fitting the clump model of Cesaroni (2019) to the SED of IRAS 20126+4104 (shown in Fig. 6), as a function of the clump surface temperature and mass. For the ;χ² calculation, we have assumed an error of 20% for all fluxes. The white dot marks the minimum corresponding to the best fit, while the white contour is the 1σ confidence level (i.e. 68% reliability) according to the criterion of Lampton et al. (1976).