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Session 110 -- Open Clusters
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[110.04] The~Reddening~and~Age~of~NGC~6791

K. A. Montgomery (Boston U.), K. A. Janes (Boston U.), R. L. Phelps (Phillips Laboratory)

NGC 6791 has long been considered to be one of the oldest open clusters in the Galaxy. However, its actual age is not known, in part because of the uncertainty in determining the foreground reddening to the cluster; previous values for E(B-V) have ranged from 0.09 to 0.23. We have determined a new value for the reddening from a combination of photoelectric and CCD photometry using the KPNO 0.9m telescopes. Broadband UBVI CCD images were taken with a 512$\times$512 CCD and included deep frames on the center, plus a 3x3 mosaic of the cluster. To determine the reddening to the cluster, stars with photoelectric photometry were individually dereddened, and the mean E(B-V) was determined to be 0.12$\pm$0.02. This value was used by Friel and Janes (1993, A\&A, 267, 75), who through spectroscopic measurements determined the cluster to be metal rich with a metallicity of [Fe/H]=0.19. Being metal rich, its UBV 2-color diagram does not match the standard solar-metallicity 2-color diagram, and a comparison with solar-metallicity isochrones only provides an upper limit of $\sim$9 Gyrs for the age for the cluster. Regardless of the choice of reddening value, if we make the assumption of solar-metallicity for the cluster we cannot simultaneously derive a consistent fit of the 2-color diagram to standard 2-color sequences or the color-magnitude diagram to published isochrones.

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