  Mg isotopic abundance ratios are measured in 20 bright red giants in globular
cluster NGC 6752 based on very high-resolution, high signal-to-noise spectra
obtained with UVES on the VLT. There is a considerable spread in the ratio
24Mg:25Mg:26Mg with values ranging from 53:9:39 to 83:10:7. We measured the
abundances of O, Na, Mg, Al, and Fe combining our sample with 21 RGB bump stars
(Grundahl et al. 2002). The abundances of the samples are consistent and
exhibit the usual anticorrelations between O-Na and Mg-Al. A positive
correlation is found between 26Mg and Al, a mild anticorrelation is found
between 24Mg and Al, while no correlation is found between 25Mg and Al. None of
the elemental or isotopic abundances show a dependence on evolutionary status
and, as shown by Gratton et al. (2001), the abundance variations exist even in
main sequence stars. This strongly suggests that the star-to-star abundance
variations are a result of varying degrees of pollution with intermediate mass
AGB stars being likely polluters. Even for the least polluted stars, the
abundances of 25Mg and 26Mg relative to 24Mg are considerably higher than
predicted for ejecta from Z=0 supernovae. Zero metallicity AGB stars may be
responsible for these higher abundances. Our measured Mg isotopic ratios reveal
another layer to the globular cluster star-to-star abundance variations that
demands extensions of our present theoretical knowledge of stellar
nucleosynthesis by giant stars.
