Submillimetre observations and modelling of Vega-type stars

We present new submillimetre observations of Vega-excess stars, and consistent modelling for all known Vega-excess stars with submillimetre data. Our analysis uses dust grain models with realistic optical properties, with the aim of determining physical parameters of the unresolved discs from just their spectral energy distributions (SEDs). For the resolved targets, we find that different objects require very different dust grain properties in order to fit the image data and SED simultaneously. Fomalhaut and Vega require solid dust grains, while HR 4796 and HD 141569 can only be fitted using porous grains. The older stars tend to have grains which are less porous than the younger stars, which may indicate that collisions in the discs have reprocessed the initially fluffy grains into a more solid form. ɛ Eri appears to be deficient in small dust grains compared with our best-fitting model. This may show that it is important to include all the factors that cause the size distribution to depart from a simple power law for grains close to the radiation pressure blow-out limit. Alternatively, this discrepancy may be due to some external influence on the disc (e.g. a planet).

When the model is applied to the unresolved targets, an estimate of the disc size can be made. However, the large diversity in dust composition for the resolved discs means that we cannot make a reliable assumption as to the composition of the grains in an unresolved disc, and there is corresponding uncertainty in the disc size. In addition, the poor fit for ɛ Eri shows that the model cannot always account for the SED even if the disc size is known. These two factors mean that it may not be possible to determine the size of a disc without actually resolving it.