MAGNESIUM ISOTOPE RATIOS IN ω CENTAURI RED GIANTS

Abstract

We have used the high-resolution observations obtained at the Anglo-Australian Telescope with Ultra-High Resolution Facility ( R ∼ 100,000) and at Gemini-S with b-HROS ( R ∼ 150,000) to determine magnesium isotope ratios for seven ω Cen 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 ω Cen 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 ω Cen and M4 show ( 25 Mg, 26 Mg)/ 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 ω Cen 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 26 Mg/ 24 Mg ratio is highest at intermediate metallicities ([Fe/H] < −1.4 dex), and for the highest [Al/Fe] values. Further, the relative abundance of 26 Mg in the extreme population stars is notably higher than that of 25 Mg, in contrast to model predictions. The 25 Mg/ 24 Mg 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.