AN ATTEMPT TO DETECT THE DUST DISK OF VEGA BY PHOTOPOLARIMETRY, AND CONSTRAINTS ON THE GRAIN-SIZE

We report on a first attempt to detect Vega's disk through optical sca ttered light by using Linear photopolarimetry. Polarization measuremen ts on the aureole of Vega were carried out between 7 '' and 30 '' from the star with a 10 '' diameter hole and down to polarization level of similar to 10(-4). No signal reliably attributable to circumstellar d ust was detected, and an upper limit to the polarized surface brightne ss of the disk is derived. This upper limit for Vega's disk is about 2 00 times lower than the polarized brightness observed around beta Pic, at an angular offset of 15 '' from the stars. The upper limit is also compared to a simple model, in which one assumes a plausible total du st mass of 2 10(-8) M., and a pole-on oriented disk with a typical rad ius of similar to 20 '' as favoured by far-infrared and submillimetric experiments. We also sup pose that the grains are spherical (Mie) par ticles. Our analysis can exclude that a major part of dust mass would consist of grains of 0.01-0.3 mu m, as in the interstellar medium. If a single size is assumed, the observational upper limit favours radii of at least 5-10 mu m or larger. If a size distribution including larg e particles (similar to 300 mu m) is assumed, the data suggests that o nly a very small fraction (similar to 1/1000) of the dust mass is in 0 .01-0.3 mu m grains.