The origin and orbit of the old, metal-rich, open cluster NGC 6791 Insights from kinematics

Context. NGC6791 is a unique stellar system among Galactic open clusters, which is at the same time one of the oldest open clusters and the most metal rich. It is located inside the solar circle, harbors a large population of binary stars, and possibly experienced prolonged star formation. The combination of all these properties is puzzling and poses the intriguing question of its origin. Aims. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outward to its current location. In this work we study the cluster's orbit and investigate the possible migration processes that may have displaced NGC 6791 to its present-day position, under the assumption that it actually formed in the inner disk. Methods. To this aim we performed integrations of NGC6791'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 NGC6791, we calculated 1000 orbits back in time for about 1 Gyr turning different non-axisymmetric components of the global potential on and off. We then compared statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 10(4) test-particles originating close to the Galactic Center (with initial galocentric radii in the range of 3-5 kpc) and undergoing radial migration during 8 Gyr of forward integration. Results. We find that a model that incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC6791 from the inner disk (galocentric radii of 3-5 kpc) to its present-day location. Such a model can provide orbital parameters that are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low.