Stromgren uvby photometry of the peculiar globular cluster NGC 2419
Stromgren uvby photometry of the peculiar globular cluster NGC 2419
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DOI：
10.1051/0004-6361/201526555
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发表时间：
2015-09
影响因子：
6.5
通讯作者：
M. Frank;A. Koch;S. Feltzing;N. Kacharov;M. Wilkinson;M. Irwin
中科院分区：
文献类型：
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作者：
M. Frank;A. Koch;S. Feltzing;N. Kacharov;M. Wilkinson;M. Irwin
NGC 2419 是一个奇特的银河球状星团，在尺寸-光度图中与其他球状星团不同，并显示出一些化学丰度异常。在这里，我们通过星团的光度测定来展示斯特罗姆格伦紫外线。使用重力和金属丰度敏感的 c(1) 和 m(1) 指数，我们确定了远远超出正式潮汐半径的可能星团成员的样本。星团非成员的估计污染仅为百分之一，使我们的目录非常适合光谱跟踪。我们推导出红巨星的光度 [Fe/H]，并且根据我们使用的文献中的金属丰度校准，我们发现对于星团平均金属丰度和单个恒星而言，与光谱 [Fe/H] 具有合理到极好的一致性。我们明确证明光度不确定性不是高斯的，在金属丰度分布函数的任何分析中都必须考虑到这一点。使用现实的非高斯模型来计算光度不确定性，我们发现正式的内部 [Fe/H] 分布 sigma = 0.11(-0.01)(+0.02) dex。这是星团真实[Fe/H]分布的上限，并且可能部分地、也可能完全地反映了光度金属丰度估计和系统效应的有限精度。单个恒星的光谱和光度 [Fe/H] 之间缺乏相关性，这进一步证明了 [Fe/H] 在 0.1 dex 水平上的分布。最后，CN 敏感的 delta(4) 以及其他颜色指数与镁丰度强烈反相关，表明第二代恒星富氮。在一些其他 CN 敏感指数中缺乏类似的相关性，支持第二代富含 He，这在这些指数中大致补偿了 CN 引起的偏移。与具有有限色散的单一连续分布相比，观察到的红巨星的 delta(4) 分布稍微更适合两个没有内部扩散的不同种群，其中氮增强的第二代恒星占恒星的 53%+/-5%。尽管有已知的特点，NGC 2419 似乎与其他贫金属银河球状星团非常相似，具有类似的氮增强第二代，并且 [Fe/H] 变化很小或没有变化，这使其与其他疑似吸积核（如 omega Cen）区分开来。 （较少的）
NGC 2419 is a peculiar Galactic globular cluster off set from the others in the size-luminosity diagram, and showing several chemical abundance anomalies. Here, we present Stromgren uvby photometry of the cluster. Using the gravity-and metallicity-sensitive c(1) and m(1) indices, we identify a sample of likely cluster members extending well beyond the formal tidal radius. The estimated contamination by cluster non-members is only one per cent, making our catalogue ideally suited for spectroscopic follow-up. We derive photometric [Fe/H] of red giants, and depending on which metallicity calibration from the literature we use, we find reasonable to excellent agreement with spectroscopic [Fe/H], both for the cluster mean metallicity and for individual stars. We demonstrate explicitly that the photometric uncertainties are not Gaussian and this must be accounted for in any analysis of the metallicity distribution function. Using a realistic, non-Gaussian model for the photometric uncertainties, we find a formal internal [Fe/H] spread of sigma = 0.11(-0.01)(+0.02) dex. This is an upper limit to the cluster's true [Fe/H] spread and may partially, and possibly entirely, reflect the limited precision of the photometric metallicity estimation and systematic effects. The lack of correlation between spectroscopic and photometric [Fe/H] of individual stars is further evidence against a [Fe/H] spread on the 0.1 dex level. Finally, the CN-sensitive delta(4), among other colour indices, anti-correlates strongly with magnesium abundance, indicating that the second-generation stars are nitrogen enriched. The absence of similar correlations in some other CN-sensitive indices supports the second generation being enriched in He, which in these indices approximately compensates the shift due to CN. Compared to a single continuous distribution with finite dispersion, the observed delta(4) distribution of red giants is slightly better fit by two distinct populations with no internal spread, with the nitrogen-enhanced second generation accounting for 53 +/- 5 per cent of stars. Despite its known peculiarities, NGC 2419 appears to be very similar to other metal-poor Galactic globular clusters with a similarly nitrogen-enhanced second generation and little or no variation in [Fe/H], which sets it apart from other suspected accreted nuclei such as omega Cen. (Less)