Red giants abundance analysis in open clusters

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.