SIMBAD references

The spectral energy distribution (SED) of circumstellar dust has been extensively used to diagnose the sizes and compositions of dust grains. We show that variations of SED slope in the long-wavelength (submillimetre to radio) regime can be used to diagnose the gross physical nature (and hence origins) of the dust, using simple geometric models that complement the use of detailed simulations. We consider two dust grain types: (i) clustered aggregates of smaller particles (monomers) and (ii) composite grains comprising ferrous inclusions within a silicate matrix. These types are intended to be analogous to fluffy cometary particles and fragments of compacted asteroids, respectively. Our results indicate that clusters of silicate grains produce a smooth SED, while composite grains with FeS inclusions produce an SED that has a pronounced drop at a wavelength an order of magnitude larger than the grain size, and is flatter at long wavelengths. As a test case, we compare the model predictions to flux measurements of the TW Hydrae disc. This SED shows a drop that only occurs in our models of compacted grains with inclusions. Since the TW Hya discs span approximately 10-40 au in radius, fluffy particles from comets were perhaps expected, as in the Sun's Kuiper belt.