Magnesium Isotopic Abundance Ratios in Cool Stars

From high resolution spectra of 61 cool dwarfs and giants, Mg isotopicabundance ratios 24Mg:25Mg:26Mg are derived from spectral synthesis of the MgHA-X lines near 5140A. Our sample spans the range -2.5 < [Fe/H] < 0.1, includingthe 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] aspredicted by recent models of Galactic chemical evolution where the Mg isotopesare produced in massive stars. A subset of kinematically identified thin diskstars have Mg isotope ratios in excellent agreement with the predictions.Within the measurement uncertainties, these thin disk stars show no scatterabout the predictions. Several of our stars are likely members of the thickdisk and their high Mg isotopic ratios may reflect the nucleosynthetic historyof the thick disk which is distinct from the predictions for, and observationsof, the thin disk. For thick disk and halo stars we find a scatter in25,26Mg/24Mg exceeding our measurement uncertainties and increasing withincreasing metallicity. Our data suggest that an additional source of 25Mg and26Mg is required. Intermediate mass asymptotic giant branch stars are likelycandidates.Comment: ApJ in press (Dec 20 2003