Chemical Evolution of Mg Isotopes versus the Time Variation of the Fine Structure Constant

We show that the synthesis of ${}^{25,26}\mathrm{M}\mathrm{g}$ at the base of the convective envelope in low-metallicity asymptotic giant branch stars can produce the isotopic ratios needed to explain the low-$z$ subset (with $z<1.8$) of the many-multiplet data from quasar absorption systems without invoking a time variation of the fine structure constant. This is supported by observations of high abundances of the neutron-rich Mg isotopes in metal-poor globular-cluster stars. We conclude that the quasar absorption spectra may be providing interesting information on the nucleosynthetic history of such systems.