[论文解读] A Search for Old Star Clusters in the Large Magellanic Cloud

D. Geisler, E. Bica+4 more|Americanae (AECID Library)|Sep 18, 1997

Astrophysics and Star Formation Studies被引用 31

一句话总结

本研究利用颜色-星等图（CMDs）和光谱数据在大麦哲伦云（LMC）中搜寻古老星团，确认了18个星团和5个候选星团。主要结果包括发现年龄长达85亿年、金属丰度低至[Fe/H] = -1.18的星团，且在密集场区的多个星团因观测限制需谨慎估计年龄和温度。

ABSTRACT

We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference $δT_1$ between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages $>1$Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.

研究动机与目标

• 利用测光和光谱数据识别并表征大麦哲伦云（LMC）中的古老星团。
• 在传统方法不可靠的密集场区，确定候选星团的年龄和金属丰度。
• 评估高场区拥挤对星团年龄和温度估计可靠性的影响。
• 比较CMD推导的参数与光谱测量结果，以检验一致性并校准方法。
• 在先前命名不完整或模糊的区域中识别潜在的古老星团候选体。

提出的方法

• 利用LMC中星团候选体的V波段及其他测光数据构建颜色-星等图（CMDs）。
• 应用T1方法从CMD形态估计星团温度和年龄，其中δT1作为关键不确定性指标。
• 利用Ca II光谱测量推导选定星团的金属丰度，尤其在测光估计不确定时。
• 通过历史命名（如Hodge & Wright、Shapley & Lindsay）和现代星表（如Bica et al.、ESO、LW、SL系统）进行星团交叉识别。
• 通过对比δT1和年龄估计与星场环境的目视检查，评估场区拥挤的影响。
• 引入已知年龄（85亿年）和金属丰度（[Fe/H] = -0.93）的控制星团ESO121SC03（KMHK1591）用于校准。

实验结果

研究问题

• RQ1 LMC中哪些星团足够古老，可被视为早期星系演化遗迹的潜在残余？
• RQ2 LMC星团的CMD年龄和金属丰度估计与Ca II光谱测量结果相比如何？
• RQ3 LMC旋臂中的场区拥挤在多大程度上影响δT1和年龄估计的可靠性？
• RQ4 LMC中星团的金属丰度和年龄分布如何？与已知控制星团相比有何差异？
• RQ5 在因拥挤导致测光数据不完整或模糊的区域，哪些星团候选体仍具可行性？

主要发现

• 本研究在LMC中识别出18个确认的古老星团，年龄范围为0.9至85亿年，其中控制星团ESO121SC03年龄最长，达85亿年。
• 光谱测量星团的金属丰度范围为[Fe/H] = -1.18（OHSC33）至-0.36（SL842），其中SL126的[Fe/H] = -1.18。
• 位于密集场区的星团（如NGC1928和NGC1939）的δT1和年龄估计不可靠，因观测限制被归类为候选体。
• SL446A被重新评估并更名，以解决与附近星团的混淆，确认其位置为α1950 = 5h24m36s，δ1950 = -67°46′18″。
• 多个星团（如SL388和SL817）显示较高的年龄估计（分别为26亿年和25亿年），表明存在显著的古老星族。
• CMD推导的年龄估计与光谱金属丰度一致，尤其在SL388（[Fe/H] = -0.76）和SL842（[Fe/H] = -0.36）等星团中表现良好，支持该方法在较不密集区域的可靠性。