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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 magnesiumisotope ratios for seven omega Centauri red giants that cover a range in ironabundance from [Fe/H] = --1.78 to --0.78 dex, and for two red giants in M4 (NGC6121). The omega Centauri stars sample both the "primordial" (i.e., O-rich, Naand Al-poor) and the "extreme" (O-depleted, Na and Al-rich) populations in thecluster. The primordial population stars in both omega Centauri and M4 show(25Mg, 26Mg)/24 Mg isotopic ratios that are consistent with those found for theprimordial population in other globular clusters with similar [Fe/H] values.The isotopic ratios for the omega Centauri extreme stars are also consistentwith those for extreme population stars in other clusters. The results for theextreme population stars studied indicate that the 26Mg/24Mg ratio is highestat intermediate metallicities ([Fe/H] < --1.4 dex), and for the highest [Al/Fe]values. Further, the relative abundance of 26Mg in the extreme population starsis notably higher than that of 25Mg, in contrast to model predictions. The25Mg/24Mg isotopic ratio in fact does not show any obvious dependence on either[Fe/H] or [Al/Fe] nor, intriguingly, any obvious difference between theprimordial and extreme population stars.

Magnesium Isotope Ratios in omega Centauri Red Giants