  From high resolution spectra of 61 cool dwarfs and giants, Mg isotopic
abundance ratios 24Mg:25Mg:26Mg are derived from spectral synthesis of the MgH
A-X lines near 5140A. Our sample spans the range -2.5 < [Fe/H] < 0.1, including
the first measurements of Mg isotope ratios in stars with metallicities below
[Fe/H]=-2.0. We confirm the decrease in 25,26Mg/24Mg with decreasing [Fe/H] as
predicted by recent models of Galactic chemical evolution where the Mg isotopes
are produced in massive stars. A subset of kinematically identified thin disk
stars have Mg isotope ratios in excellent agreement with the predictions.
Within the measurement uncertainties, these thin disk stars show no scatter
about the predictions. Several of our stars are likely members of the thick
disk and their high Mg isotopic ratios may reflect the nucleosynthetic history
of the thick disk which is distinct from the predictions for, and observations
of, the thin disk. For thick disk and halo stars we find a scatter in
25,26Mg/24Mg exceeding our measurement uncertainties and increasing with
increasing metallicity. Our data suggest that an additional source of 25Mg and
26Mg is required. Intermediate mass asymptotic giant branch stars are likely
candidates.
