CHEMICAL-COMPOSITION OF OPEN CLUSTERS .1. FE/H FROM HIGH-RESOLUTION SPECTROSCOPY

The abundance ratio Fe/H is the key indicator of chemical composition and chemical evolution of the Galaxy. An age-metallicity relation has been found to exist for older disk stars (e.g., Twarog); here we have looked at stars in clusters younger than a few billion years. With "precision" global Fe/H abundances we have looked at the age-metallicity relation at younger eras and at the extent, time scale and homogeneity of mixing in the Galactic disk. Our sample includes F dwarfs in the α Per, the Pleiades, and the Hyades clusters, and the UMa, Hyades, and Wolf 630 moving groups, and a selection of bright F field dwarfs; the objects span the age range from 5 × 107 to 2 × 109 yr. We have made high-resolution, high signal-to-noise (S/N) spectroscopic observations in three spectral regions: 6540-6655 Å, 7048-7182 Å, and 7740-7852 Å. There are 15 Fe I lines with well-determined gf-values in these regions. The observations were obtained at the coudé focus at the CFHT with a Reticon detector and at the Hale 200 inch (5.08 m) with a CCD detector at resolutions of 0.11 and 0.21 Å, respectively. Abundances of Fe/H were determined for each line with a model atmosphere/abundance routine. The means, [〈Fe/H〉], were found for each star and then the global cluster means. The global means are estimated to be accurate to typically ±0.04 dex. We find that the intrinsic dispersion in the Fe abundances in each cluster or group is extremely small; the observed dispersion about the mean cluster Fe abundances is consistent with observational error alone. There is no evidence of a trend in [Fe/H] with age for these young clusters and groups, but there are clear differences in [Fe/H] among the groups. There must be intrinsic differences in the metal content in the local gas out of which these groups were formed. Clusters of the same age have metallicities differing from each other by amounts well beyond the errors. The time scale for mixing in the disk exceeds ∼109 yr. Although clusters of different ages may have different metallicities, there is no pattern to it. Age and [Fe/H] values for the older visual binaries (∼2 × 109) also show no age-metallicity relation.