Abundances in Turn-off Stars in the Old, Metal-Rich Open Cluster, NGC 6791

Subject headings: stars: abundances; stars: evolution; stars: late-type; stars:

Population II; open clusters and associations: NGC 6791

The study of open clusters has greatly advanced our understanding of stellar evolution through comparisons between observations and theoretical models. The distribution of stars in the HR diagram has been an especially useful tool in this regard. Comparing clusters of different ages via the turnoff points, as shown initially by e.g. Johnson & Sandage (1955), Sandage (1956), has proven to be extremely important. Open clusters have also been used to increase our knowledge of cluster and Galactic dynamics, the distance scale of the universe, and the chemical history of the Galaxy.

Stars within a particular cluster are thought to have been formed together, within a few Myr, and share the same composition and space motion. Comparative composition studies have mostly been limited to Fe and Li, however. See Friel et al. (2002) for a presentation of metallicities and kinematics for 39 old open clusters (older than 0.8 Gyr). However, it is important to study a full range of elements including CNO and other alpha-elements, Fe-peak elements, n-capture elements. Cluster-to-cluster comparisons can reveal possible trends with age. Comparisons between dwarfs and giants reveal potential mixing that could alter the surface abundances in the evolved giants. Such comparisons have been done for globular clusters providing interesting insights into their evolution, but open clusters have not yet received such comprehensive studies. Old clusters are especially interestesting as they are nearly as old as the Galactic disk.

Ages can be determined for open clusters and they are found to span the lifetime of the Galactic disk. Open clusters that are old are relatively rare because old clusters are subject to dissipation after several passages through the disk plane such that former cluster members become part of the general field. The field stars of Chen et al. (2003) and Feltzing & Gonzales (2001) that are old and metal-rich could be the remnants of similar, but less massive, clusters like NGC 6791. The extant old clusters tend to probe the outer regions of the Galaxy and are good tracers of Galactic chemical evolution. Current evidence seems to point to a rapid, early chemical enrichment of the disk, plus infall of some metal-poor gas (e.g. Friel et al. 2002). This picture comes from abundances of Fe only. A much fuller picture would require abundances for an array of elements since different mass ranges of stars produce different nucleosynthetic products.

The old open clusters contain the clues to early chemical evolution in the atmospheres of their unevolved stars. Unlike many of the giant stars, their atmospheres are not contaminated by the products of nuclear reactions in the interiors.

The cluster, NGC 6791, is an enigmatic open cluster. It is unusually massive; apparently it’s metallicity is at least a factor of two higher than the sun’s, yet it is very old. In addition, it has extreme kinematics and is about 1 kpc above the Galactic plane. This cluster defies many of the existing paradigms about the formation of the Galaxy and its chemical history. Thus it is especially important to determine the details of its chemical composition.

NGC 6791 is anomalous in several respects: (1) Typical open clusters contain a few hundred stars, but NGC 6791 is very populous with a mass of ∼4000 M ⊙ (Carraro et al. 2006, Gratton et al. 2006). (2) It is apparently super metal-rich with [Fe/H] ∼+0.4 -but this has been determined only from its evolved stars (Peterson & Green 1998, Worthey & Jowett 2003, Gratton et al. 2006, Carraro et al. 2006). (3) In spite of being metal-rich, it is very old; the photometry is excellent (Montgomery et al. 1994) and the HR diagrams yield ages of 8-10 Gyr (Demarque et al. 1992, Garnavich et al. 1994, Montgomery et al. 1994, Chaboyer et al. 1999). Other old open clusters include M67 and NGC 188 which have solar metallicity and ages of 4-7 Gyr. (4) It clearly does not fit any age-metallicity relation for the Galactic disk given its very old age and unusually high metallicity (e.g. Carraro et al. 2006).

(5) The combination of its heliocentric distance of ∼4 kpc (King et al. 2005) and its Galactic latitude of +11 • , means it is ∼1 kpc above the Galactic plane, far above the disk where open clusters reside. (6) Given its high metallicity and its distance from the Galactic center (Bedin et al. 2006), it does not fit any radial abundance gradient for the Galaxy (e.g Carraro et al. 2006). (

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