Sodium content as a predictor of the advanced evolution of globular cluster stars

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Abstract

The asymptotic giant branch (AGB) phase is the final stage of nuclear burning for low-mass stars. Although Milky Way globular clusters are now known to harbour (at least) two generations of stars1,2, they still provide relatively homogeneous samples of stars that are used to constrain stellar evolution theory3,4,5. It is predicted by stellar models that the majority of cluster stars with masses around the current turn-off mass (that is, the mass of the stars that are currently leaving the main sequence phase) will evolve through the AGB phase6,7. Here we report that all of the second-generation stars in the globular cluster NGC 6752—70 per cent of the cluster population—fail to reach the AGB phase. Through spectroscopic abundance measurements, we found that every AGB star in our sample has a low sodium abundance, indicating that they are exclusively first-generation stars. This implies that many clusters cannot reliably be used for star counts to test stellar evolution timescales if the AGB population is included. We have no clear explanation for this observation.

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Figure 1: Sample selection in the Strömgren uvby colour–magnitude diagram of NGC 6752.
Figure 2: Sodium abundance results for NGC 6752.
Figure 3: Theoretical stellar model tracks overlain on the Strömgren colour–magnitude diagram of NGC 6752.

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Acknowledgements

We thank Y. Momany of the European Southern Observatory (ESO, Chile) for providing his UBV photometric data set, which is mentioned in Supplementary Information section 2. S.W.C. acknowledges support from the Australian Research Council’s Discovery Projects funding scheme (project DP1095368). R.J.S. is the recipient of a Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation. F.G. acknowledges funding for the Stellar Astrophysics Centre provided by The Danish National Research Foundation. The research was supported by the ASTERISK project funded by the European Research Council (grant agreement no. 267864). This work was based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 089.D-0038 (principal investigator S.W.C.) and made extensive use of the SIMBAD, Vizier, 2MASS and NASA ADS databases.

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S.W.C. designed and prepared the ESO Very Large Telescope (VLT) observing proposal, collected the spectroscopic data, and prepared the paper. V.D. reduced and analysed the spectroscopic data, and prepared the paper. D.Y. designed and prepared the ESO/VLT observing proposal and assisted in the paper preparation. T.N.C. calculated the stellar models and prepared figures for the paper. J.C.L. assisted in the preparation of the observing proposal and with the paper preparation. R.J.S., G.C.A. and E.C.W. assisted in the paper preparation and made preliminary observations with the Anglo-Australian Telescope. F.G. provided the uvby photometric data for the AGB and red giant branch sample and assisted in the paper preparation.

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Correspondence to Simon W. Campbell.

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This file contains Supplementary Table 1, Supplementary Discussion and Supplementary References. (PDF 184 kb)

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Campbell, S., D’Orazi, V., Yong, D. et al. Sodium content as a predictor of the advanced evolution of globular cluster stars. Nature 498, 198–200 (2013). https://doi.org/10.1038/nature12191

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