Le séminaire a lieu le Lundi
5 mars 2001 de 14h30 à 15h30
dans la salle de réunion
du PHC (Observatoire de Nice).
The existence of narrow rings around planets as different
as Saturn,
Uranus and Neptune, suggests that these rings may occur
under more general
circumstances. We consider the system of planetary rings
with shepherds as
a restricted four-body problem, and as first approximation,
neglect the
interactions among ring particles. We show that narrow
rings and selection
mechanisms actually exist generically in the case of
rotating short-range
potentials (both attractive and repulsive). Due to the
hierarchical
arrangement of the periodic orbits, stable orbits consequence
saddle-center bifurcations show up. In configuration
space these lead to
patterns similar to planetary rings. The generic character
of this
mechanism, as well as the application to 1/r potentials
will be
addressed.