Magnesium Isotope Ratios in omega Centauri Red Giants
  We have used high resolution observations obtained at the AAT with UHRF (R ~
100,000) and at Gemini-S with b-HROS (R ~ 150,000) to determine magnesium
isotope ratios for seven omega Centauri red giants that cover a range in iron
abundance from [Fe/H] = --1.78 to --0.78 dex, and for two red giants in M4 (NGC
6121). The omega Centauri stars sample both the "primordial" (i.e., O-rich, Na
and Al-poor) and the "extreme" (O-depleted, Na and Al-rich) populations in the
cluster. The primordial population stars in both omega Centauri and M4 show
(25Mg, 26Mg)/24 Mg isotopic ratios that are consistent with those found for the
primordial population in other globular clusters with similar [Fe/H] values.
The isotopic ratios for the omega Centauri extreme stars are also consistent
with those for extreme population stars in other clusters. The results for the
extreme population stars studied indicate that the 26Mg/24Mg ratio is highest
at intermediate metallicities ([Fe/H] < --1.4 dex), and for the highest [Al/Fe]
values. Further, the relative abundance of 26Mg in the extreme population stars
is notably higher than that of 25Mg, in contrast to model predictions. The
25Mg/24Mg isotopic ratio in fact does not show any obvious dependence on either
[Fe/H] or [Al/Fe] nor, intriguingly, any obvious difference between the
primordial and extreme population stars.
