Variation with the effective temperature of the optical depth, characterizing various evolutionary phases of a 1 M_⊙ model star, from the tip of the AGB (first point on the plane) to the post-AGB. Full squares connected with solid lines indicate the values of obtained with the physical ingredients discussed in Sect. 2 to model the evolution of the star, whereas open squares refer to the expected optical depths that would be observed in the present epoch, . Full and open diamonds refer to the results obtained by assuming a different scaling of mass loss with the effective temperature during the contraction to the post-AGB phase (see text for details). Green pentagons indicate the optical depths obtained by artificially increasing the mass-loss rate at the tip of AGB by a factor of three. The dashed orange line indicates the optical depth of J053250.69, obtained by SED fitting.