Frank Spahn, Institut f. Physik u. Astronomie, Universität Potsdam Title: What have "Propellers" in Saturns Rings to do with Planet-Formation? Time: February 11, 17:15 Place: Building 28, Room 0.008, Campus Golm, Universitaet Potsdam Abstract: We investigate the action of gravitational perturbers in thin cold astrophysical discs. The model includes viscous diffusion of the disc matter and gravitational scattering by the perturber as counteracting processes. Two types of density structures are found, depending on the mass of the perturbing body and on the amount of momentum transport in the disc. A gap around the whole circumference of the disc is opened if the perturber is more massive than a certain threshold. Alternatively, a local S-shaped density modulation - a "propeller" - is generated. Beyond the perturber's mass, the kinematic viscosity of the disk comprises the crucial parameter of the model describing the momentum transport in the disc. Analytical and numerical solutions provide the characteristic spatial extent of the "propeller" to depend on the mass of the perturber and the disk-viscosity. These theoretical results are applied to dense planetary rings perturbed by an embedded moonlet. The inspection of the Cassini-imaging data revealed meanwhile >100 "propellers" in the 100-metres size range to reside in Saturn's A ring. Interestingly - these moonlets jostle in three narrow radial bands in the A ring. The mere existence of such moonlets has crucial consequences for the origin scenario of the rings. Another exciting application concerns gas-dust discs around a protostar perturbed by a protoplanet - practically the "nursery of a planet". Again either gaps or "propeller"-shaped structures can be expected to have formed within the disk. With increasing resolution of modern telescopes the chance might appear in future to be witness of a planetary growth by studying structures in circum-stellar disks.