Heavy-Element Abundances of the Old Open Cluster NGC 6791*

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Published 1998 July 1 © 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation Ruth C. Peterson and Elizabeth M. Green 1998 ApJ 502 L39 DOI 10.1086/311486

1538-4357/502/1/L39

Abstract

The old open cluster NGC 6791 is believed to be more metal-rich than any other, yet several hot blue horizontal-branch (BHB) stars are probable members. We have performed an abundance analysis of the coolest BHB candidate, 2-17, whose proper motion and radial velocity both support cluster membership. Its luminosity and its low rotational velocity, vsini=16 ± 1 km s-1, suggest that it is a BHB star rather than a very luminous blue straggler. We find an effective temperature Teff=7300 ± 50 K, gravity logg=3.6 ± 0.3 dex, and an iron abundance more than twice solar, [Fe/H]=+0.4 ± 0.1 dex. This result is free from the serious line blending and continuum suppression that hamper spectroscopic analyses of metal-rich giants. The Ca abundance is in the solar proportion with respect to iron, ruling out A-star peculiarities in 2-17. The relative abundances of C, O, and Al are nearly solar; mixing effects are probably small but not ruled out. The light elements Mg and Si are enhanced, N and Na especially so, as seen in metal-rich galaxies. NGC 6791 thus provides an excellent template for the study of the stellar content of metal-rich galactic systems. Both its abundances and its continued existence suggest that metal-rich systems might generally form in locally deep potential wells. Further study of its hot population should clarify the mechanisms responsible for producing hot stars in a metal-rich environment and thus assist the interpretation of the integrated light and the ultraviolet upturn in elliptical galaxies.

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Footnotes

  • Based on observations obtained at the Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Associated Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.

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10.1086/311486