Magnesium isotope ratios in Hyades stars

Using classical model atmospheres and an LTE analysis, Mg isotope ratios Mg:Mg:Mg are measured in 32 Hyades dwarfs covering 4000K ≤ Teff ≤ 5000K. We find no significant trend in any isotope ratio versus Teff and the mean isotope ratio is in excellent agreement with the solar value. We determine stellar parameters and Fe abundances for 56 Hyades dwarfs covering 4000K ≤ Teff ≤ 6200K. For stars warmer than 4700K, we derive a cluster mean value of [Fe/H] = 0.16 ± 0.02 (σ = 0.1), in good agreement with previous studies. For stars cooler than 4700K, we find that the abundance of Fe from ionized lines exceeds the abundance of Fe from neutral lines. At 4700K [Fe/H]II−[Fe/H]I ≃ 0.3 dex while at 4000K [Fe/H]II−[Fe/H]I ≃ 1.2 dex. This discrepancy between the Fe abundance from neutral and ionized lines likely reflects inadequacies in the model atmospheres and the presence of Non-LTE or other effects. Despite the inability of the models to reproduce ionization equilibrium for Fe, the Mg isotope ratios appear immune to these problems and remain a powerful tool for studying Galactic chemical evolution. Subject headings: Galaxy: Open Clusters and Associations: Individual: Name: Hyades, Stars: Abundances Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.