NGC 6791: An Exotic Open Cluster or the Nucleus of a Tidally Disrupted Galaxy?

We report on high resolution Echelle spectroscopy of 20 giant stars in theGalactic old open clusters NGC 6791 obtained with Hydra at the WIYN telescope.High precision radial velocity allow us to isolate 15 {\it bona fide} clustermembers. From 10 of them we derive a global [M/H]=+0.39$\pm$0.05. We thereforeconfirm that NGC 6791 is extremely metal rich, exhibits a few marginallysub-solar abundance ratios, and within the resolution of our spectra does notshow evidences of spread in metal abundance. With these new data we re-derivethe cluster fundamental parameters suggesting that it is about 8 Gyr old and4.3 kpc far from the Sun. The combination of its chemical properties, age,position, and Galactic orbit hardly makes NGC 6791 a genuine Population I opencluster. We discuss possible interpretations of the cluster peculiaritiessuggesting that the cluster might be what remains of a much larger system,whose initial potential well could have been sufficient to produce highmetallicity stars, and which has been depopulated by the tidal field of theGalaxy. Alternatively, its current properties may be explained by theperturbation of the Galactic bar on an object originated well inside the solarring, where the metal enrichment had been very fast.Comment: 29 pages, 10 eps figure, accepted for publication in the Astrophysical Journa