Sketch of the protostellar core adopted in this model, including a central source characterised by the bolometric luminosity (L_*) and the stellar temperature (T_*). The inner zone is characterised by the sublimation threshold of dust grains (T_sub) and the threshold radius (r_threshold). The thin shell of hot dust is determined by the shell temperature (T_shell) and its radius (R_shell). The outer region is given by the outer radius (r_out).