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 the Galactic old open cluster NGC 6791, obtained with Hydra at the WIYN telescope. High-precision radial velocity allows us to isolate 15 bona fide cluster members. From 10 of them we derive a global [M/H] = +0.39 ± 0.05. We therefore confirm that NGC 6791 is extremely metal-rich, exhibits a few marginally subsolar abundance ratios, and within the resolution of our spectra does not show evidence of spread in metal abundance. With these new data we rederive the fundamental cluster parameters, suggesting that it is about 8 Gyr old and 4.3 kpc from the Sun. The combination of its chemical properties, age, position, and Galactic orbit hardly makes NGC 6791 a genuine Population I open cluster. We discuss possible interpretations of the cluster peculiarities, suggesting that the cluster might be what remains of a much larger system whose initial potential well could have been sufficient to produce high-metallicity stars and which has been depopulated by the tidal field of the Galaxy. Alternatively, its current properties may be explained by the perturbation of the Galactic bar on an object that originated well inside the solar ring, where the metal enrichment was very fast.

[1]  M. S. Roberts,et al.  The Hot Stars of Old Open Clusters: M67, NGC 188, and NGC 6791 , 1998, astro-ph/9804164.

[2]  G. Carraro,et al.  On the Galactic disc age–metallicity relation , 1998 .

[3]  S. Cassisi,et al.  The White Dwarf Cooling Sequence of NGC 6397 , 2005 .

[4]  S. Cassisi,et al.  Reaching the End of the White Dwarf Cooling Sequence in NGC 6791 , 2008, 0801.1346.

[5]  To Appear in ApJ Preprint typeset using L ATEX style emulateapj v. 6/22/04 THE EFFECTIVE TEMPERATURE SCALE OF FGK STARS. II. Teff: COLOR: [Fe/H] CALIBRATIONS , 2008 .

[6]  P. Demarque,et al.  Solar calibration and the ages of the old disk clusters M67, NGC 188, and NGC 6791 , 1992 .

[7]  Y.-W. Lee,et al.  Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture , 2001 .

[8]  E. Friel,et al.  Kinematics of the Old Open Cluster System , 1995 .

[9]  S. Cassisi,et al.  Color-Magnitude Diagrams and Luminosity Functions Down to the Hydrogen-Burning Limit. III. A Preliminary Hubble Space Telescope Study of NGC 6791 , 2005 .

[10]  James Liebert,et al.  The Age, Extinction, and Distance of the Old, Metal-rich Open Cluster NGC 6791 , 1998, astro-ph/9812097.

[11]  E. Friel,et al.  RADIAL VELOCITIES OF MORE OLD OPEN CLUSTERS , 1989 .

[12]  Giampaolo Piotto,et al.  Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement , 2004, astro-ph/0412016.

[13]  Robert L. Kurucz,et al.  Model Atmospheres for Population Synthesis , 1992 .

[14]  Giovanni Carraro,et al.  IS THE GALACTIC DISC OLDER THAN THE HALO , 1999 .

[15]  E. Friel,et al.  Abundances of Red Giants in the Old Open Cluster Collinder 261 , 2003 .

[16]  P. Garnavich,et al.  Red-giant branch of the old, metal-rich open cluster NGC 6791 , 1994 .

[17]  L. Girardi,et al.  Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 , and from to 0.03 , 1999, astro-ph/9910164.

[18]  Mario Mateo,et al.  DWARF GALAXIES OF THE LOCAL GROUP , 1998, astro-ph/9810070.

[19]  Sabine Moehler,et al.  14th European Workshop on White Dwarfs , 2005 .

[20]  Peter B. Stetson,et al.  Homogeneous Photometry. III. A Star Catalog for the Open Cluster NGC 6791 , 2003 .

[21]  B. Carney,et al.  Infrared Photometry of NGC 6791 , 2004, astro-ph/0411014.

[22]  Santiago Arribas,et al.  The effective temperature scale of giant stars (F0–K5) - II. Empirical calibration of versus colours and [Fe/H] , 1999 .

[23]  V. Straižys,et al.  Fundamental stellar parameters derived from the evolutionary tracks , 1981 .

[24]  D. Geisler.  RADIAL VELOCITIES OF STARS IN FIVE OLD OPEN CLUSTERS , 1988 .

[25]  M. Grenon.  The Kinematics and Origin of smr Stars , 1999 .

[26]  E. Green,et al.  Heavy-Element Abundances of the Old Open Cluster NGC 6791 , 1998 .

[27]  M. Fukugita,et al.  A Study of the B−V Color-Temperature Relation , 1999, astro-ph/9904299.

[28]  Optical photometry and spectral classification in the field of the open cluster ngc 6996 in the north america nebula , 2004, astro-ph/0401492.

[29]  Jong-Hak Woo,et al.  Y2 Isochrones with an Improved Core Overshoot Treatment , 2004 .

[30]  T. Kinman.  The Star Cluster NGC 6791. , 1965 .

[31]  S. Cassisi,et al.  A Large Stellar Evolution Database for Population Synthesis Studies. I. Scaled Solar Models and Isochrones , 2004, astro-ph/0405193.

[32]  G. Piotto,et al.  Color-Magnitude Diagram and Luminosity Function of M4 near the Hydrogen-burning Limit , 2001, astro-ph/0109059.

[33]  B. J. Taylor.  Statistics and supermetallicity: The metallicity of NGC 6791 , 2001 .

[34]  Nathan D. Miller,et al.  Metallicities of Old Open Clusters , 2002 .

[35]  G. Carraro,et al.  Metal Abundances in Extremely Distant Galactic Old Open Clusters. I. Berkeley 29 and Saurer 1 , 2004, astro-ph/0406679.