The origin and orbit of the old, metal-rich, open cluster NGC 6791:
  Insights from kinematics
  NGC 6791 is a unique stellar system among Galactic open clusters being at the
same time one of the oldest open clusters and the most metal rich. Combination
of its properties is puzzling and poses question of its origin. One possible
scenario is that the cluster formed close to the Galactic Center and later
migrated outwards to its current location. In this work we study the cluster's
orbit and investigate the possible migration processes which might have
displaced NGC 6791 to its present-day position, under the assumption that it
actually formed in the inner disk. To this aim we performed integrations of NGC
6791's orbit in a potential consistent with the main Milky Way parameters. In
addition to analytical expressions for halo, bulge and disk, we also consider
the effect of bar and spiral arm perturbations, which are expected to be very
important for the disk dynamical evolution, especially inside the solar circle.
Starting from state-of-the art initial conditions for NGC 6791, we calculate
1000 orbits back in time for about 1 Gyr turning on and off different
non-axisymmetric components of the global potential. We then compare
statistical estimates of the cluster's recent orbital parameters with the
orbital parameters of 10^4 test-particles originating close to the Galactic
Center (having initial galocentric radii in the range of 3-5 kpc) and
undergoing radial migration during 8 Gyr of forward integration. We find that a
model which incorporates a strong bar and spiral arm perturbations can indeed
be responsible for the migration of NGC 6791 from the inner disk (galocentric
radii of 3-5 kpc) to its present-day location. Such a model can provide orbital
parameters which are close enough to the observed ones. However, the
probability of this scenario as it results from our investigations is very low.
