Sublimation distances of silicate (top panel) and carbonaceous material (bottom panel) in composite grains (C+Si or C+vacuum respectively) as a function of grain size in the Fomalhaut environment, with either a constant (dashed line) or a size-dependent (solid line) sublimation temperature. Relevant disk properties obtained from preliminary modeling are used (Sect. 4). Size-dependent sublimation temperatures are a function of the size-dependent survival timescales shown in Fig. 3 causing jumps at the material-dependent blowout limit.