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

Insights from kinematics

¹ ESO, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
e-mail: ljilkova@eso.org; gcarraro@eso.org
² Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
³ Dipartimento di Astronomia, Universitá di Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
⁴ Astronomical Institute, Academy of Sciences of the Czech Republic, Boční II 1401/1a, 141 31 Prague, Czech Republic
e-mail: bruno@ig.cas.cz
⁵ Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 121 16 Prague, Czech Republic
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: iminchev@aip.de

Received: 25 May 2011
Accepted: 1 March 2012

Abstract

Context.NGC 6791 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 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 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 10⁴ 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 NGC 6791 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.