Mg isotopic ratios in giant stars of the globular cluster NGC 6752

• Published in: Astronomy and astrophysics (Berlin) 2003-05, Vol.402 (3), p.985-1001
• Main Authors: Yong, D., Grundahl, F., Lambert, D. L., Nissen, P. E., Shetrone, M. D.
• Format: Article
• Language: English
• Subjects: galaxies: clusters: general; galaxies: clusters: individual: NGC 6752; stars: abundances; stars: evolution; stars: fundamental parameters
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20030296

Mg isotopic abundance ratios are measured in 20 bright red giants in globular cluster NGC 6752 based on very high-resolution ($R\sim110\,000$), 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. Consideration of the extremes of the abundance variations provides the composition of the ambient material and the processed material. In the least contaminated stars (lowest Na and Al and highest O and Mg abundances), we infer a Mg isotopic ratio around 80:10:10 and a composition ([O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe]) $\simeq$ (0.6, -0.1, 0.5, 0.0). In the most polluted stars, we find a Mg isotopic ratio around 60:10:30 and a composition ([O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe]) $\simeq$ (-0.1, 0.6, 0.3, 1.2). 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.