The isotopic abundances of magnesium in stars

Isotopic abundance ratios Mg-24:Mg-25:Mg-26 are derived for 20 stars from high-resolution spectra of the MgH A-X 0-0 band at 5140 Angstrom. With the exception of the weak G-band giant HR 1299, the stars are dwarfs that sample the metallicity range - 1.8 < [Fe/H] < 0.0. The abundance of Mg-25 and Mg-26 relative to the dominant isotope Mg-24 decreases with decreasing [Fe/H] in fair accord with predictions from a recent model of Galactic chemical evolution in which the Mg isotopes are synthesized by massive stars. Several stars appear especially enriched in the heavier Mg isotopes, suggesting contamination by material from the envelopes of intermediate-mass asymptotic giant branch stars.