The puzzle of metallicity and multiple stellar populations in the globular clusters in Fornax
The puzzle of metallicity and multiple stellar populations in the globular clusters in Fornax
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天炉座球状星团中的金属丰度和多恒星族群之谜
DOI：
10.1093/mnras/stt1057
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发表时间：
2013
影响因子：
4.8
通讯作者：
M. Criscienzo
中科院分区：
文献类型：
--
作者：
F. D’Antona;V. Caloi;A. D'ercole;M. Tailo;E. Vesperini;P. Ventura;M. Criscienzo
我们检查了Fornax星系团的光度数据，将注意力集中在它们的水平分支颜色分布上，如果有的话，还关注RR LYR变量的分数和周期分布。基于我们对HB形态在多个恒星世代背景下氦含量变化的理解，我们证明了星系团F2、F3和F5必须包含大量的第二代恒星(~54-65%)。在一个简单的化学演化模型的基础上，我们证明了初始质量为当前质量的~4到~10倍的模型可以再现氦在这些团簇中的分布。具有非常短的第二代恒星形成幕的模型也可以重现观测到的氦的分布，但需要更大的初始质量，最高可达当前质量的20倍。虽然这一可能的初始GC质量范围的下限与低金属丰度场星的观测结果一致，但我们也讨论了场星的金属丰度标度(基于CaII三重态光谱)与Fornax星团的金属丰度可能不一致的可能性。要再现F2、F3、F5中的Hb形态，需要两个有趣的假设：1)第一代Hb星都是红色的。根据这一解释，位于Fornax场中的低金属丰度恒星，其颜色比RRLyRS的蓝色((V-I)o<；=0.3或(B-V)o<；=0.2)更蓝的Hb，应该是星团中诞生的第二代恒星；对Fornax场现有颜色测量的初步分析提供了~20%的蓝色Hb星，处于低金属丰度范围；2)单个第二代红巨星的质量损失是太阳质量的几个百分点，比第一代恒星的质量损失大。
We examine the photometric data for Fornax clusters, focussing our attention on their horizontal branch color distribution and, when available, on the RR Lyr variables fraction and period distribution. Based on our understanding of the HB morphology in terms of varying helium content in the context of multiple stellar generations, we show that clusters F2, F3 and F5 must contain substantial fractions of second generation stars (~54-65%). On the basis of a simple chemical evolution model we show that the helium distribution in these clusters can be reproduced by models with cluster initial masses ranging from values equal to ~4 to ~10 times larger than the current masses. Models with a very short second generation star formation episode can also reproduce the observed helium distribution but require larger initial masses up to about twenty times the current mass. While the lower limit of this range of possible initial GC masses is consistent with those suggested by the observations of the low metallicity field stars, we also discuss the possibility that the metallicity scale of field stars (based on CaII triplet spectroscopy) and the metallicities derived for the clusters in Fornax may not be consistent with each other. The reproduction of the HB morphology in F2,F3,F5 requires two interesting hypotheses: 1) the first generation HB stars lie all at "red" colours. According to this interpretation, the low metallicity stars in the field of Fornax, populating the HB at colours bluer than the blue side ((V-I)o<=0.3 or (B-V)o<=0.2) of the RR Lyrs, should be second generation stars born in the clusters;a preliminary analysis of available colour surveys of Fornax field provides a fraction ~20% of blue HB stars, in the low metallicity range; 2) the mass loss from individual second generation red giants is a few percent of a solar mass larger than the mass loss from first generation stars.