From high-resolution spectra of 61 cool dwarfs and giants, Mg isotopic abundance ratios ~(24)Mg: ~(25)Mg: ~(26)Mg are derived from spectral synthesis of the MgH A-X lines near 5140 A. 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)Mg/~(24)Mg and ~(26)Mg/~(24)Mg with decreasing [Fe/H], as predicted by recent models of Galactic chemical evolution in which 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)Mg/~(24)Mg and ~(26)Mg/~(24)Mg exceeding our measurement uncertainties and increasing with increasing metallicity. Our data suggest that an additional source of ~(25)Mg and ~(26)Mg is required. Intermediate-mass asymptotic giant branch stars are likely candidates.
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