Hubble Space Telescope Photometry of the Fornax Dwarf Spheroidal Galaxy: Cluster 4 and Its Field
Hubble Space Telescope Photometry of the Fornax Dwarf Spheroidal Galaxy: Cluster 4 and Its Field
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哈勃太空望远镜对天炉座矮球状星系的光度测量：星团 4 及其场
DOI：
10.1086/301034
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
R. Zinn
中科院分区：
文献类型：
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作者：
R. Buonanno;C. Corsi;M. Castellani;G. Marconi;F. F. Pecci;R. Zinn
利用哈勃空间望远镜的观测资料，我们绘制了天炉座矮球状星系中球状星团4及其周围区域的色星等图。这些图表延伸到主层序的转折点以下，并得出了人口年龄的测量结果。天炉座星群的CMD最显著的特征是一个密集的红色水平支（HB）恒星团，一个广阔的红巨星分支（RGB），以及一个跨越大范围光度的主序星。在这个CMD中，在天炉座的五个球状星团的HB位置上只有很少的恒星，这表明只有很小一部分的恒星在年龄和化学成分上与星团相似。主序星的亮度跨度很大，这表明星星的形成始于12亿年前，并持续到10.5亿年前。CMD中有独立的亚巨星分支，这表明星星的形成不是连续的，而是发生在爆发中。簇4的CMD具有陡峭的RGB，由此我们估计[Fe/H]=-2.0。这是相当低的估计，从综合轻的集群，这种差异的起源进行了讨论。聚类4具有非常红的HB，因此是第二参数效应的主要示例。将星团4与其他天炉座星团和M68（一个银河系晕中非常缺乏金属的球状星团）进行比较，发现星团4比其他星团年轻2.3亿年，而其他星团的HB更蓝。这种年龄差异与年龄是不确定性范围内的第二个参数的预测一致。星团4的CMD与银河系光晕Ruprecht 106中不寻常的球状星团几乎相同，这表明它们具有非常相似的年龄和化学成分。我们讨论了星团4也类似于R106的可能性，即它的[Fe/H]比R106的陡峭RGB所显示的要高，而[α/Fe]比R106的一些最近的观测结果所显示的要比球状星团的通常值低。五个天炉座星团的CMD表明星团年龄是一个主要的，但可能不是唯一的第二个参数。Buonanno等人最近得出结论，簇密度可能影响簇1、2、3和5的HB形态。尽管中心密度差异很大，但簇4和R106的HB非常相似。这表明，密度可能作为第二个参数，在具有HB的集群中，这些HB处于向蓝色移动的边缘，或者由于另一个原因已经是蓝色的，例如非常老的年龄。
Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dwarf spheroidal galaxy and its surrounding field. These diagrams extend below the main-sequence turnoffs and have yielded measurements of the ages of the populations. The most prominent features of the CMD of the Fornax field population are a heavily populated red clump of horizontal-branch (HB) stars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the positions of the HBs of the five globular clusters in Fornax, which suggests that only a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main sequence suggests that star formation began in the field ≃12 Gyr ago and continued to ≃0.5 Gyr ago. There are separate subgiant branches in the CMD, which indicates that the star formation was not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H] ≃ -2.0. This is considerably lower than estimates from the integrated light of the cluster, and the origins of this discrepancy are discussed. Cluster 4 has a very red HB and is, therefore, a prime example of the second-parameter effect. Comparisons of cluster 4 with the other Fornax clusters and with M68, a very metal-poor globular cluster of the Galactic halo, reveal that cluster 4 is ≃3 Gyr younger than these other clusters, which have much bluer HBs. This age difference is consistent with the prediction that age is the second parameter to within the uncertainties. The CMD of cluster 4 is virtually identical to that of the unusual globular cluster of the Galactic halo Ruprecht 106, which suggests that they have very similar ages and chemical compositions. We discuss the possibility that cluster 4 also resembles R106 in having a higher [Fe/H] than is indicated by its steep RGB and also a lower [α/Fe] ratio than is usual for a globular cluster, as indicated by some recent observations of R106. The CMDs of the five Fornax clusters indicate that cluster age is a major—but probably not the sole—second parameter. Buonanno et al. recently concluded that cluster density probably influenced the HB morphologies of clusters 1, 2, 3, and 5. Despite a very large difference in central density, the HBs of cluster 4 and R106 are very similar. This suggests that density may act as a second parameter in clusters that have HBs that are on the verge of moving toward the blue or are already blue for another reason, such as very old age.