On the HR 4796 A circumstellar disk

We investigate in details the properties of the disk surrounding thr 8 Myr old star HR 4796 A, one of the few stars bringing precious clues to better understand the scenario which lead to planetary system formation. We propos e a model able to reproduce all the available observations: the full spectr al energy distribution from the mid-infrared to the millimeter wavelengths, resolved scattered light and thermal emission observations. We show that t he circumstellar matter splits into two dust components: a cold annulus, pe aked at 70 AU from the star, made of ISM-like grains (amorphous composition , porosity similar to 0.6) larger than 10 mu m and a population of hot dust close to the star (at about 9 AU) made of comet-like grains (crystalline c omposition, porosity similar to 0.97). Both dust populations are highly col lisional and the grain size distribution in the cold annulus is found to be cut-off by radiation pressure. At 70 AU, bodies as large as a few meters a re required to fit the data leading to a minimum disk mass of a few Earth m asses and to a gas to dust ratio less than 1. We discuss aftewards some imp lications on the disk structure and effects of larger bodies.