Magnesium Isotope Ratios in Hyades Stars

Using classical model atmospheres and an LTE analysis, Mg isotope ratios super(24)Mg: super(25)Mg: super(26)Mg are measured in 32 Hyades dwarfs covering effective temperatures 4000 K < T sub(eff) < 5000 K. We find no significant trend in any isotope ratio versus T sub(eff), 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 4000 K < T sub(eff) < 6200 K. For stars warmer than 4700 K, we derive a cluster mean value of [Fe/H] = 0.16 plus or minus 0.02 ( capital sigma = 0.1), in good agreement with previous studies. For stars cooler than 4700 K, we find that the abundance of Fe from ionized lines exceeds the abundance of Fe from neutral lines. At 4700 K, [Fe/H] sub(II)-[Fe/H] sub(I) ~ 0.3 dex, while at 4000 K [Fe/H] sub(II)-[Fe/H] sub(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 the ionization equilibrium for Fe, the Mg isotope ratios appear immune to these problems and remain a powerful tool for studying Galactic chemical evolution.