Heavy-Element Abundances of the Old Open Cluster NGC 6791

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, vsin${r sin}$ -->${r sin}$ -->i=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${r eff}$ -->$t SUBgt {r eff}t/SUBgt $ -->=7300 ± 50 K, gravity log${r log}$ -->${r log}$ -->g=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.