Sub-mm observations and modelling of Vega type stars

We present new sub-mm observations of Vega excess stars, and consistent modelling for all known Vega excess stars with sub-mm data. Our analysis uses dust grain models with realistic optical properties, with the aim of determining physical parameters of the unresolved disks from just their SEDs. For the resolved targets, we find that different objects require very different dust grain properties in order to simultaneously fit the image data and SED. Fomalhaut and Vega require solid dust grains, whilst HR4796 and HD141569 can only be fitted using porous grains. The older stars tend to have less porous grains than younger stars, which may indicate that collisions in the disks have reprocessed the initially fluffy grains into a more solid form. ǫ Eri appears to be deficient in small dust grains compared to our best fitting model. This may show that it is important to include all the factors which cause the size distribution to depart from a simple power law for grains close to the radiation pressure blowout limit. Alternatively, this discrepancy may be due to some external influence on the disk (e.g. a planet). When the model is applied to the unresolved targets, an estimate of the disk size can be made. However, the large diversity in dust composition for the resolved disks means that we cannot make a reliable assumption as to the composition of the grains in an unresolved disk, and there is corresponding uncertainty in the disk size. In addition, the poor fit for ǫ Eri shows that the model cannot always account for the SED even if the disk size is known. These two factors mean that it may not be possible to determine a disk’s size without actually resolving it.