Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991, and 7762

Abstract

We have analysed high-dispersion Echelle spectra (R = 60 000) of red giant members of five open clusters to derive abundances for many elements from Na to Eu. The [Fe/H] values are −0.06 ± 0.03 for Stock 2, −0.11 ± 0.03 for NGC 2168, −0.01 ± 0.03 for NGC 6475, 0.00 ± 0.03 for NGC 6991, and −0.07 ± 0.03 for NGC 7662. Sodium is enriched in the giants relative to the abundance expected of main-sequence stars of the same metallicity. This enrichment of [Na/Fe] by about +0.25 attributed to the first dredge-up is discussed in the light of theoretical predictions and recently published abundance determinations. Abundance ratios [El/Fe] for other elements are with very few exceptions equal to those of field giants and dwarfs, i.e. [El/Fe] ≃ 0.00 for [Fe/H] ∼0.0. An exception is the overabundance of La, Ce, Nd, and Sm in NGC 6991 but this is consistent with our previous demonstration that the abundances of these s-process products vary by about ±0.2 among clusters of the same [Fe/H], a variation found also among field giants and dwarfs.