Variations in chemical abundances with evolutionary phase have been identified among stars in globular and open clusters with a wide range of metallicities. In the metal-poor clusters, these variations compare well with predictions from stellar structure and evolution models considering the internal diffusive motions of atoms and ions, collectively known as atomic diffusion, when moderated by an additional mixing process with a fine-tuned efficiency. We present here an investigation of these effects in the Galactic globular cluster NGC 6121 (M4) ([Fe/H] = -1.13) through a detailed chemical abundance analysis of 86 stars using high-resolution ESO/VLT FLAMES spectroscopy. The stars range from the main-sequence turnoff point (TOP) to the red giant branch (RGB) just above the bump. We identify C-N-O and Mg-Al-Si abundance anti-correlations, and confirm the presence of a bimodal population differing by 1 dex in nitrogen abundance. The composition of the second-generation stars imply pollution from both massive (20-40 Msol) and asymptotic giant branch stars. We find evolutionary variations in chemical abundances between the TOP and RGB, which are robust to uncertainties in stellar parameters and modelling assumptions. The variations are weak, but match predictions well when employing efficient additional mixing. Without correcting for Galactic production of lithium, we derive an initial lithium abundance 2.63+-0.10, which is marginally lower than the predicted primordial BBN value.
Multiple Populations of the Large Magellanic Cloud Globular Cluster NGC 2257: No Major Environmental Effect on the Formation of Multiple Populations of the Old Globular Clusters in Large Magellanic Cloud
本研究通过 Ca–CN–CH 测光分析年老贫金属 LMC 球状星团 NGC 2257 的多族群，发现其族群比例、金属丰度及元素变化趋势与银河系普通球状星团一致，表明宿主星系环境对多族群形成演化无重大影响。
Isochrone Fitting of Galactic Globular Clusters -- VIII. Homogeneous estimates of parameters for 27 clusters
Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences（俄罗斯科学院中央（普尔科沃）天文台）；Institute for Nuclear Research, Russian Academy of Sciences（俄罗斯科学院核物理研究所）；Saint Petersburg State University（圣彼得堡国立大学）；Departamento de Astronomia, Instituto de Física, UFRGS（阿雷格里港联邦大学天文学系、物理学院）；Faculty of Physics, Moscow State University（莫斯科国立大学物理系）
Globular Cluster Abundances from High-Resolution, Integrated-Light Spectroscopy. II. Expanding the Metallicity Range for Old Clusters and Updated Analysis Techniques
Impact of the low solar abundance on the ages of globular clusters
本文介绍了Asplund及其合作者（2004）关于低太阳丰度对基于等时线拟合的最古老星团年龄的影响的研究结果。我们已经建立了新的恒星模型和相应的等时线使用这种新的太阳混合适当的太阳校准。我们发现使用Asplund et al.（2004）的金属丰度会导致银河系中老球状星团的典型年龄大约增加10%。虽然这可能看起来很小，但它对解释银河系球状星团的形成时代有重要影响。Asplund等人（2004）的丰度可能不一定会威胁到目前的和谐宇宙学，但会表明银河系球状星团在再电离之前和主星系体开始形成之前形成。这与目前对星系形成的理解形成对比。
