Contribution Contributed Talk
How small are the smallest dust grains in debris discs and why?
The minimum size of dust grains in debris discs is a sensitive indicator of a variety of physical processes operating in these systems. We analysed a sample of 32 Herschel-resolved debris discs to derive the minimum grain size and found an intriguing trend of that size changing with the luminosity of the debris disc host star. The trend is statistically significant and is pretty robust against variations of the assumed dust chemical composition and porosity. The effect goes beyond a simple explanation that grains around more luminous stars have to be larger in order not to be blown out by the stronger radiation pressure of such stars. Instead, we show that the trend can be related to the microphysics of collisions that replenish the visible dust. Alternatively or additionally, the trend can be explained by assuming that debris discs of more luminous stars have higher dynamical excitations than those of less luminous primaries. If true, this would imply that the protoplanetary progenitors of debris discs around the stars of earlier spectral types were more successful in producing massive stirrers, be it big planetesimals of planets. As a by-product of our analysis, we suggest a recipe of how to estimate the true radii of spatially unresolved debris discs, based solely on their spectral energy distribution.