大麦云星云中中等年龄星团的人口参数。I. NGC 1846及其宽主序列关闭
Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff
摘要：
哈勃太空望远镜上的高级巡天照相机已被用来获取大麦云星云中的中等年龄星团NGC 1846的深层高分辨率图像。我们提出了基于F435 W、F555 W和F814 W成像的新的颜色星等图（CMD）。我们测试以前观察到的广泛的主序列关闭区域的“污染”的场星和（演变）双星系统。我们发现，虽然这些影响了该区域中的对象数量，但没有一个可以完全解释大的颜色扩散。因此，我们的研究结果巩固了最近的发现，在这个星团的主序关闭区域的恒星有一个大的颜色分布，这是无关的测量误差或污染的领域的明星，可能是由于一个~ 3亿年的年龄范围内的星团恒星。一个无偏估计的恒星密度分布在主序星的转折区表明，蔓延是相当连续的，而不是强烈的双峰如前所述。我们适合的CMD与几个不同的理论等时线，并确定系统的不确定性人口参数时，使用任何一组等时线。我们注意到年龄和[alpha/Fe]之间的简并，这可以通过匹配CMD中完整红巨星分支的形状（曲率）来提升。我们发现，在主序星关闭区的上部的恒星比在CMD的任何其他地区，包括更大质量的红巨星分支和渐近巨星分支恒星更集中。我们考虑了几种可能的形成方案，这些方案解释了在NGC 1846的CMD中观察到的不寻常特征。
英文摘要：
The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the previously observed broad main sequence turnoff region for "contamination" by field stars and (evolved) binary star systems. We find that while these impact the number of objects in this region, none can fully account for the large color spread. Our results therefore solidify the recent finding that stars in the main sequence turnoff region of this cluster have a large spread in color which is unrelated to measurement errors or contamination by field stars, and likely due to a ~300 Myr range in the ages of cluster stars. An unbiased estimate of the stellar density distribution across the main sequence turnoff region shows that the spread is fairly continuous rather than strongly bimodal as suggested previously. We fit the CMDs with several different sets of theoretical isochrones, and determine systematic uncertainties for population parameters when derived using any one set of isochrones. We note a degeneracy between age and [alpha/Fe], which can be lifted by matching the shape (curvature) of the full red giant branch in the CMD. We find that stars in the upper part of the main sequence turnoff region are more centrally concentrated than those in any other region of the CMD, including more massive red giant branch and asymptotic giant branch stars. We consider several possible formation scenarios which account for the unusual features observed in the CMD of NGC 1846.
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