  Evidence from a large sample of quasar absorption-line spectra in damped
Lyman-alpha systems has suggested a possible time variation of the fine
structure constant alpha. The most statistically significant portion of this
sample involves the comparison of Mg and Fe wavelength shifts using the
many-multiplet (MM) method. However, the sensitivity of this method to the
abundance of heavy isotopes, especially Mg, is enough to imitate an apparent
variation in alpha in the redshift range 0.5 < z < 1.8. We implement recent
yields of intermediate mass (IM) stars into a chemical evolution model and show
that the ensuing isotope distribution of Mg can account for the observed
variation in alpha provided the early IMF was particularly rich in intermediate
mass stars (or the heavy Mg isotope yields from AGB stars are even higher than
in present-day models). As such, these observations of quasar absorption
spectra can be used to probe the nucleosynthetic history of low-metallicity
damped Lyman-alpha systems in the redshift range 0.5 < z < 1.8. This analysis,
in conjunction with other abundance measurements of low-metallicity systems,
reinforces the mounting evidence that star formation at low metallicities may
have been strongly influenced by a population of IM stars. Such IM stars have a
significant influence on other abundances, particularly nitrogen. We constrain
our models with independent measurements of N, Si, and Fe in damped Lyman-alpha
systems as well as C/O in low-metallicity stars. In this way, we obtain
consistent model parameters for this chemical-evolution interpretation of the
MM method results.
