Magnesium Isotope Ratios in Hyades Stars

Using classical model atmospheres and an LTE analysis, Mg isotope ratios24Mg:25Mg:26Mg are measured in 32 Hyades dwarfs covering 4000K < Teff < 5000K.We find no significant trend in any isotope ratio versus Teff and the meanisotope ratio is in excellent agreement with the solar value. We determinestellar 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 (sigma=0.1), in good agreement with previous studies. For starscooler than 4700K, we find that the abundance of Fe from ionized lines exceedsthe abundance of Fe from neutral lines. At 4700K [Fe/H]_II - [Fe/H]_I = 0.3 dexwhile at 4000K [Fe/H]_II - [Fe/H]_I = 1.2 dex. This discrepancy between the Feabundance from neutral and ionized lines likely reflects inadequacies in themodel atmospheres and the presence of Non-LTE or other effects. Despite theinability of the models to reproduce ionization equilibrium for Fe, the Mgisotope ratios appear immune to these problems and remain a powerful tool forstudying Galactic chemical evolution.Comment: ApJ in press (March 10 2004