  Using classical model atmospheres and an LTE analysis, Mg isotope ratios
24Mg: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 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 (sigma=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.
