GCs的渐近巨分支：仅选择进入
The Asymptotic Giant Branches of GCs: Selective Entry Only
摘要：
对银河系球状星团中AGB星的少数观测表明，GC AGB星群主要由氰弱星组成。这与RGB（和其他）群体中的分布形成强烈对比，RGB（和其他）群体中的CN谱带强度通常显示50：50双峰。如果AGB星族的分布确实有很大的不同，那么这就给低质量恒星演化理论带来了一个严重的问题，因为从RGB到AGB的这种表面丰度变化是恒星模型无法预测的。然而，这只是一个初步结论，因为它是基于非常小的AGB样本量。为了测试这个问题是否真的存在，我们进行了一项专门针对GC中AGB星的观测活动。我们利用AAT（2df/AAOmega）多目标摄谱仪获得了9个银河系GC中约250颗AGB星的中分辨率光谱。我们介绍了GC第二个参数三重奏：NGC 288、NGC 362和NGC 1851研究的一些初步结果。结果确实表明，在AGB上缺乏具有强CN带的恒星。为了证实这一现象的可靠性，而不仅仅局限于CN带强度及其变化，我们利用FLAMES/VLT对NGC 6752进行了观测，并给出了一些初步结果。我们的钠丰度结果最终表明，只有GC中的一部分恒星经历了AGB演化阶段。这是对这一现象的第一次直接、具体的证实。
英文摘要：
The handful of available observations of AGB stars in Galactic Globular Clusters suggest that the GC AGB populations are dominated by cyanogen-weak stars. This contrasts strongly with the distributions in the RGB (and other) populations, which generally show a 50:50 bimodality in CN band strength. If it is true that the AGB populations show very different distributions then it presents a serious problem for low mass stellar evolution theory, since such a surface abundance change going from the RGB to AGB is not predicted by stellar models. However this is only a tentative conclusion, since it is based on very small AGB sample sizes. To test whether this problem really exists we have carried out an observational campaign specifically targeting AGB stars in GCs. We have obtained medium resolution spectra for about 250 AGB stars across 9 Galactic GCs using the multi-object spectrograph on the AAT (2df/AAOmega). We present some of the preliminary findings of the study for the second parameter trio of GCs: NGC 288, NGC 362 and NGC 1851. The results indeed show that there is a deficiency of stars with strong CN bands on the AGB. To confirm that this phenomenon is robust and not just confined to CN band strengths and their vagaries, we have made observations using FLAMES/VLT to measure elemental abundances for NGC 6752.We present some initial results from this study also. Our sodium abundance results show conclusively that only a subset of stars in GCs experience the AGB phase of evolution. This is the first direct, concrete confirmation of the phenomenon.
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