β ratio of a dust grain as a function of its size in the Fomalhaut environment. A 50–50 carbon-silicate mixture is assumed and depicted by the lower curve while the upper curve considers half porous carbon grains. The vertical and horizontal lines mark the β = 0.5 (blowout size for grains released by parent-bodies on initially circular orbits) and β = 1.0 (blowout size for any initial orbit) limits, before and after the sublimation of silicates.