Silicate dust in a Vega-excess system

The 10-µm spectrum of the K5V star SAO 179815 ( = HD 98800) is presented, and conclusively demonstrates the presence of small silicate dust grains (probably in a disc) around this star. The 9.7-µm silicate dust feature is unusually broad and shallow in this system. This, together with the slow fall-off of flux at longer wavelengths, constrains the size and density distributions of dust grains in models of the disc. We find that there must be a significant population of small grains (radii at least as small as 0.01 µm), as well as a population of large grains (radii at least as large as 100 µm) in order to explain all the observed properties of the disc.