Multiple stellar populations in Magellanic Cloud clusters - I. An ordinary feature for intermediate age globulars in the LMC?
Multiple stellar populations in Magellanic Cloud clusters - I. An ordinary feature for intermediate age globulars in the LMC?
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DOI：
10.1051/0004-6361/200810870
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发表时间：
2008-10
影响因子：
6.5
通讯作者：
A. Milone;L. Bedin;G. Piotto;Jay Anderson
中科院分区：
文献类型：
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作者：
A. Milone;L. Bedin;G. Piotto;Jay Anderson
上下文在大质量星团NGC 2808和半人马座欧米茄中发现了多个主序（MS），沿着在NGC 1851和NGC 6388中发现了多个亚巨星分支，这挑战了长期以来的范式，即球状星团由简单的恒星种群组成。最近对大麦哲伦云（LMC）中期年龄星团的测光研究进一步复杂了这一演变过程，发现其中的主序关闭（MSTO）是双峰的（NGC 1806和NGC 1846）或加宽的（NGC 1783和NGC 2173）。目标。我们进行了一项研究的档案HST图像的大，小麦哲伦星系团的频率测量的集群与证据的多个或长期的星星形成事件，并确定其主要属性的目的。我们发现了涵盖广泛年龄的53个集群的有用图像。本文分析了16个中等年龄（~1-3 Gyr）大麦哲伦星系团的色星等图（CMD）。方法.减少了数据，并对光度测定法进行了微分红化校正（如需要）。我们发现，11个集群表现出异常的传播（或分裂）的颜色和大小周围的MSTO，即使其他主要功能的CMD（MS，红巨星分支，渐近巨星分支）是狭窄的和水平的分支（HB）红团很好地定义。通过使用的CMD的恒星的区域，围绕集群，我们证明了所观察到的功能是明确相关的集群。我们使用人工星测试来证明，传播（或分裂）是不是由于光度误差或二进制的文物。结果我们确认，两个集群（NGC 1806和NGC 1846）清楚地表现出两个不同的MSTO和观察，第一次，在NGC 1751双MSTO。在这三个星团中，与较亮的MSTO相对应的星群包括了星团中超过三分之二的恒星星群。我们确认了NGC 1783中存在多个恒星种群。我们的测光强烈地表明，这个集群的MSTO是由两个不同的分支。在7个星团（ESO 057-SC 075、HODGE 7、NGC 1852、NGC 1917、NGC 1987、NGC 2108和NGC 2154）中，我们观察到了MSTO的内在加宽，这可能表明这些星团经历了一个漫长的星星形成时期，其时间跨度在1.5亿至2.5亿年之间。IC 2146、NGC 1644、NGC 1652、NGC 1795和NGC 1978的CMD在我们的光度精度范围内没有显示出扩散或双峰性的证据。总而言之，我们的样本中有70 ± 25%与简单的单一恒星种群假设不一致。
Context. The discovery of multiple main sequences (MS) in the massive clusters NGC 2808 and Omega Centauri, along with multiple subgiant branches in NGC 1851 and NGC 6388 has challenged the long-held paradigm that globular clusters consist of simple stellar populations. This evolving picture has been further complicated by recent photometric studies of the Large Magellanic Cloud (LMC) intermediate-age clusters, where the main sequence turn-off (MSTO) was found to be bimodal (NGC 1806 and NGC 1846) or broadened (NGC 1783 and NGC 2173). Aims. We undertook a study of archival HST images of Large and Small Magellanic Cloud clusters with the aim of measuring the frequency of clusters with evidence of multiple or prolonged star formation events and determining their main properties. We found useful images for 53 clusters that cover a wide range of ages. In this paper, we analyse the color-magnitude diagrams (CMD) of sixteen intermediate-age (~1–3 Gyr) LMC clusters. Methods. The data were reduced and the photometry corrected for differential reddening (where required). We find that eleven clusters show an anomalous spread (or split) in colour and magnitude around the MSTO, even though the other main features of the CMD (MS, red giant branch, asymptotic giant branch) are narrow and the horizontal branch (HB) red clump well defined. By using the CMD of the stars in regions that surround the cluster, we demonstrate that the observed feature is unequivocally associated to the clusters. We use artificial-star tests to demonstrate that the spread (or split) is not an artifact due to photometric errors or binaries. Results. We confirm that two clusters (NGC 1806 and NGC 1846) clearly exhibit two distinct MSTOs and observe, for the first time, a double MSTO in NGC 1751. In these three clusters, the population corresponding to the brighter MSTO includes more than two-thirds of the cluster stellar population. We confirm the presence of multiple stellar populations in NGC 1783. Our photometry strongly suggests that the MSTO of this cluster is formed by two distinct branches. In seven clusters (ESO057-SC075, HODGE7, NGC 1852, NGC 1917, NGC 1987, NGC 2108, and NGC 2154), we observed an intrinsic broadening of the MSTO that may suggest that these clusters have experienced a prolonged period of star formation that spans a period between 150 and 250 Myr. The CMDs of IC 2146, NGC 1644, NGC 1652, NGC 1795, and NGC 1978 show no evidence of spread or bimodality within our photometric precision. In summary, 70 ± 25% of our sample are not consistent with the simple, single stellar population hypothesis.