The Disk of ? Pictoris in the Light of Polarimetric Data

We model the linear polarization of the radiation of b Pic scattered by dust particles in the circumstel- lar disk. The observed spatial distribution and the wavelength dependence of the polarization together with the colors of the b Pic disk require that particles in a wide size range be present in the disk, with the grains smaller than a few microns in size being somewhat depleted but still of importance for the polarization and colors. The inferred size distribution is consistent with the production and loss mecha- nisms: the sourcespresumably collisions and evaporation of large bodiescontinuously produce dust with a power-law size distribution with the exponent D3.5 over a broad range of sizes, but the particles smaller than a few microns are blown away by the radiation pressure, which shortens the time they spend in the disk and decreases their number densities. Compact (or slightly porous) silicates are found to give better agreement with the observations, although other materials are still not ruled out and a high —uffiness of the large particles is possible. The observed asymmetry in the polarization of two wings can be explained if more small grains (by 20%¨30%) are present on the northeast side of the disk. We show that such an asymmetry in the size distributions in two wings might be caused by an in—uence of the interstellar medium; a required amount of small grains could be produced by destructive collisions of interstellar grains with the circumstellar dust particles. Subject headings: circumstellar matterpolarizationstars: individual (b Pictoris)