Author(s): , ,
Institution(s): 1. Indiana University, 2. University of Hawaii
Stellar evolution and Galactic evolution have both been greatly advanced by the study of star clusters. In addition the elemental abundance results from clusters have revealed information about Galactic chemical evolution and nucleosynthesis. The cluster, NGC 6791, has a number of bizarre properties that make it especially interesting for comparative cluster studies. It is old (8.3 Gyr) yet metal-rich ([Fe/H] = +0.30). It has a heliocentric distance of 4 kpc and a galactic latitude of +11 degrees which makes it 1 kpc above the galactic plane. Its boxy orbit has a high eccentricity (~0.5) with a perigalactic distance of 3 kpc and an apogalactic distance of 10 kpc. The orbital period of ~130 Myr indicates that it has crossed the Galactic plane several times yet has remained as an intact cluster. We have determined abundances from high-resolution (R = 46,000) Keck/HIRES spectra of turn-off stars in this open cluster NGC 6791. We have a solid determination of [Fe/H] = +0.30 +/-0.02 from measurements of some 40 unblended, unsaturated lines of both Fe I and Fe II in eight turn-off stars. Our O abundances come from the O I triplet near 7774 \AA\ and are corrected for small nLTE effects. We find consistent ratios of [O/Fe]n with a mean of -0.06 +/-0.02, indicating a single population of stars. Our results for the alpha elements [Mg/Fe], [Si/Fe], [Ca/Fe] and [Ti/Fe] are near solar and compare well with those of old, metal-rich field stars. The Fe-peak elements, Cr and Ni, have values of [Cr/Fe] = +0.05 +/-0.02 and [Ni/Fe] = +0.04 +/-0.01. Determinations of upper limits were found for Li by spectrum synthesis; this is consistent with the upper limits in this temperature range for turn-off/subgiant stars in the relatively old, super-metal-rich cluster NGC 6253. We speculate that no stars in NGC 6791 have retained the Li with which they formed.