氟丰度与球状星团化学史之谜
Fluorine abundances and the puzzle of globular cluster chemical history
摘要：
在银河系的几个球状星团中，氟的丰度因恒星而异。这些意想不到的化学性质进一步证实了在几个GC中已经发现的化学不均匀性，可能是由这些系统中形成的第一代恒星造成的。这篇文章的目的是补充我们的理解的F-行为的GC星，并寻找新的限制其多个恒星星族的形成历史。我们收集了15个RGB恒星的近红外光谱，这些恒星属于GC，金属丰度范围很广：47 Tuc，M4，NGC 6397和M30。F，Na和Fe的丰度已估计光谱合成。对于样品中最富金属的簇（47 Tuc），没有发现F和Na丰度之间的关系。在这个GC中，RGB恒星确实在[F/Fe]上表现出相当小的差异，而不像在[Na/Fe]比率上发现的较大差异。这揭示了一个相当不均匀的恒星系统和复杂的化学演化历史47 Tuc。在M4中，我们研究的一颗星证实了另一个小组在2005年报告的Na-F分布。对于两个非常贫金属的GC（NGC 6397和M30），只有上限的F丰度已被导出。我们表明，只有在发现意想不到的富F巨星和/或满足特殊的观测条件下，才能用现有的望远镜和摄谱仪在这种贫金属GC中估计F丰度。GC中F和Na丰度的分布表明，它们的RGB成员似乎属于两个分离良好的区域。到目前为止，在不同的GC中分析的所有RGB恒星确实都被发现是富F Na贫或富F Na贫。这种分离良好的双峰制度是一致的独立的形成情节怀疑在大多数星系GC。
英文摘要：
The abundance of fluorine in a few Galactic globular clusters is known to strongly vary from star-to-star. These unexpected chemical properties are an additional confirmation of the chemical inhomogeneities already found in several GC, and probably caused by the first generations of stars formed in these systems. The aim of this article is to complement our understanding of the F-behaviour in GC stars and to look for new constraints on the formation histories of their multiple stellar populations. We have collected near-IR spectra of 15 RGB stars belonging to GC spanning a wide range of metallicity: 47 Tuc, M4, NGC6397 and M30. F, Na and Fe abundances have been estimated by spectral synthesis. No anticorrelation between F and Na abundances are found for the most metal-rich cluster of the sample (47 Tuc). In this GC, RGB stars indeed exhibit rather small differences in [F/Fe] unlike the larger ones found for the [Na/Fe] ratios. This reveals a rather inhomogeneous stellar system and a complex chemical evolution history for 47 Tuc . In M4, one star of our study confirms the previous Na-F distribution reported by another group in 2005. For the two very metal-poor GC (NGC6397 and M30), only upper limits of F abundances have been derived. We show that F abundances could be estimated in such metal-poor GC with current telescopes and spectrographs only if unexpected F-rich giants are found and/or exceptional observational conditions are met. The distribution of the F and Na abundances in GC reveal that their RGB members seem to belong to two well-separated regions. All the RGB stars analysed so far in the different GC are indeed found to be either F-rich Na-poor or F-poor Na-rich. Such well-separated bimodal regimes are consistent with the separate formation episodes suspected in most galactic GC.
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