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

¹ Department of Astronomy, University of Texas, Austin, TX 78712, USA
² Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark e-mail: fgj,pen@phys.au.dk
³ McDonald Observatory, University of Texas, HC 75, Box 1337 MCD, Fort Davis, TX 79734, USA e-mail: shetrone@astro.as.utexas.edu

Corresponding author: D. Yong, tofu,dll@astro.as.utexas.edu

Received: 13 January 2003
Accepted: 27 February 2003

Abstract

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. 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]) (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]) (-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 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.