Monash Centre for Astrophysics, School of Mathematical Sciences, Monash University, Melbourne, Victoria 3800, Australia.
Nature. 2013 Jun 13;498(7453):198-200. doi: 10.1038/nature12191. Epub 2013 May 29.
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 stars, they still provide relatively homogeneous samples of stars that are used to constrain stellar evolution theory. 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 phase. 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.
渐近巨星分支（AGB）阶段是低质量恒星核燃烧的最后阶段。尽管现在已知银河系球状星团中至少存在两代恒星，但它们仍然提供了相对均匀的恒星样本，用于约束恒星演化理论。恒星模型预测，大多数质量在当前截止质量（即目前正在离开主序阶段的恒星质量）左右的星团恒星将经历 AGB 阶段。在这里，我们报告说，球状星团 NGC 6752 中的所有第二代恒星——占星团总数的 70%——都未能达到 AGB 阶段。通过光谱丰度测量，我们发现我们样本中的每一颗 AGB 恒星的钠丰度都很低，这表明它们都是第一代恒星。这意味着，如果包括 AGB 星族，许多星团可能无法可靠地用于恒星计数来测试恒星演化时间尺度。我们对这一观察结果没有明确的解释。
