|
The radii of debris discs and the minimum size of their dust grains are sensitive indicators of a variety of
physical processes operating in these systems. We analysed a sample of ~30 Herschel-resolved debris discs to
derive the radii and the minimum grain size and found an intriguing trend of that grain 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.
|
