International Astronomical Union Symposi Cosmic Chemical Evolution: Proceedings of the 187th Symposium of the International Astronomical Union, Held at Kyoto, Ja, Book 187, (Paperback)

The open cluster NGC 6791 is now considered both the oldest and the most metal-rich known. Its age is 8 -10 Gyrs, twice as old as the canonical solar-metallicity cluster M67 (Garnavich et al. 1994; Demarque, Green, & Guenther 1992; Tripicco et al. 1995). That its metallicity is significantly above solar is suggested from moderate-resolution spectroscopy and from a mismatch of its color-magnitude diagram (CMD) with solar-metallicity isochrones. Tripicco et al. (1995) find [Fe/H] = +0.27 to +0.44. The cluster population is rich. In addition to about a dozen red giants and two dozen red horizontal-branch stars, the cluster has several very hot HB stars (Kaluzny & Udalski 1992). Liebert et al. (1994) have shown that the extremely blue stars are mostly sdB/sdO stars and at least 3 or 4 are likely cluster members, the first ever discovered in an open cluster. These may provide the key to the puzzling upturn in ultraviolet flux below 1500A seen in many high-metallicity systems (Burstein et al. 1988; Ferguson et al. & Liebert 1993).