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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


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.

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