Multi-color photometry of the Galactic globular cluster M 75 = NGC 6864. A new sensitive metallicity indicator and the position of the horizontal branch in UV ,
Multi-color photometry of the Galactic globular cluster M 75 = NGC 6864. A new sensitive metallicity indicator and the position of the horizontal branch in UV ,
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银河球状星团 M 75 = NGC 6864 的多色光度测定。一种新的敏感金属丰度指示器和 UV 中水平分支的位置，
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
Santiago
中科院分区：
文献类型：
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作者：
V. Kravtsov;G. Alcaíno;G. Marconi;F. I. D. Astronomia;Ucn;A. S. A. Institute;Msu;Moscow Isaac Newton Institute;Santiago European Southern Observatory;Santiago
目标。我们对UBVI中的银河系球状星团(GC)M75进行了新的多色光度测量，重点研究了颜色-星等图(CMD)中较亮的序列，特别是它们在U-CMD中的位置。具体地说，我们研究了水平(HB)和红巨型分支(RGB)相对于U震级主序关闭(TO)的水平。方法：研究方法。除了M75的测光，我们还使用了我们收集的贫金属区GC的测光数据，[Fe/H]ZW<；−1.1Dex，这些数据来自不同设备的观测，但由观测到的星系团中的标准恒星校准。结果。我们确认了我们早先的发现，并将其扩展到更大的震级范围。我们证明了用NTT/ESO望远镜的EMMI相机观测到的贫金属气团的U星等差的∆U BHb比用拉斯坎帕纳斯天文台的100��望远镜和1.3m华沙望远镜观测到的气团要小约0.4-0.5mag。同时，∆U RGB TO，TO和RGB拐点(最亮)之间的U等差，并不表现出对所使用的U滤光片特性的明显依赖，但它取决于团簇的金属丰度。我们首次证明了参数∆U RGB与[Fe/H]的依赖关系，并通过假定参数与金属丰度之间的线性关系，估计了它的解析表达式。它的斜率∆U RGB/∆[Fe/H]∼1.2mag/dex，比RGB凸起位置在V星等中的位置与金属丰度的关系大约陡峭两倍。文中还讨论了M75的渐近巨枝(AGB)团和RGB光度函数(LF)的特征。
Aims. We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M 75 in UBVI and focus on the brighter sequences of the color–magnitude diagram (CMD), with particular emphasis on their location in U-based CMD. Specifically, we study the level both of the horizontal (HB) and red giant branches (RGB) relative to the main-sequence turnoff (TO) in the U magnitude. Methods. Along with the presented photometry of M 75, we use our collection of photometric data on GCs belonging to the metalpoor range, [Fe/H]ZW < −1.1 dex, obtained from observations with different equipment, but calibrated by standard stars situated in the observed cluster fields. Results. We confirm our earlier finding, and extend it to a larger magnitude range. We demonstrate that ∆U BHB expressing the difference in U magnitude between the TO point and the level of the blue HB, near its red boundary, of the metal-poor GCs observed with the EMMI camera of the NTT/ESO telescope is about 0.4–0.5 mag smaller as compared to GCs observed with the 100 �� telescope and 1.3 m Warsaw telescope of the Las Campanas Observatory. At the same time, ∆U RGB TO , the difference in U magnitude between the TO and RGB inflection (brightest) points, does not show such an apparent dependence on the characteristics of U filters used, but it depends on cluster metallicity. We have shown, for the first time, the dependence of the parameter ∆U RGB on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, ∆U RGB /∆[Fe/H] ∼ 1.2 mag/dex, is approximately a factor of two steeper than that of the dependence of the RGB bump position in the V magnitude on metallicity. The asymptotic giant branch (AGB) clump and features of the RGB luminosity function (LF) of M 75 are also discussed.