The Isotopic Abundances of Magnesium in Stars

Isotopic abundance ratios ²⁴Mg:²⁵Mg:²⁶Mg are derived for 20 stars from high-resolution spectra of the MgH A-X 0-0 band at 5140 Å. 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 ²⁵Mg and ²⁶Mg relative to the dominant isotope ²⁴Mg 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.