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arXiv 0704.2445astro-ph
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银河系球状星团M75 = NGC 6864的多色测光。一种新的灵敏金属丰度指示剂及其在UV中水平分支的位置

Multi-Color Photometry of the Galactic Globular Cluster M75 = NGC 6864. A New Sensitive Metallicity Indicator and the Position of the Horizontal Branch in UV

V. Kravtsov, G. Alcaino, G. Marconi, F. Alvarado

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9 pages, 7 figures, accepted for publication in A&A
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摘要：

我们对银河系球状星团M75进行了UBVI多色测光，重点分析了色星等图（CMD）中较亮的序列，特别强调了它们在U基CMD中的位置。具体来说，我们研究的水平（HB）和红巨星分支（RGB）相对于主序关（TO）的U级。除了M75的光度测量外，我们还利用了我们收集的属于贫金属区的GC的光度测量数据，[Fe/H]zw<-1.1 dex，这些数据是用不同的设备观测得到的，但用位于观测星团场中的标准星进行了校准。我们证实了我们先前的发现，并将其推广到更大的星等范围，我们证明了表示TO点和蓝色HB水平之间的U星等差的DeltaU_{TO} ^{BHB}，在它的红色边界附近，用NTT/ESO望远镜的EMMI照相机观测到的贫金属GC比用100“望远镜观测到的GC小0.4-0.5等。望远镜和1.3米华沙望远镜的Las Campanas天文台。同时，Delta U_{TO} ^{RGB}，即TO和RGB拐点（最亮）之间的U幅度差异，并不显示出对所使用的U滤波器特性的明显依赖性，但它取决于簇金属丰度。我们第一次证明了参数DeltaU_{TO} ^{RGB}对[Fe/H]的依赖性，并通过假设参数与金属丰度之间的线性关系，估计了其解析表达式。其斜率Δ U_{TO} ^{RGB}/Δ [Fe/H]~1.2 mag/dex约为。比V量值中RGB凸块位置对金属性的依赖性陡两倍。讨论了M_（75）的渐近巨星支（AGB）团和RGB光度函数（LF）的特征。

英文摘要：

We carry out and analyze new multi-color photometry of the Galactic globular cluster (GC) M75 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. Along with the presented photometry of M75, we use our collection of photometric data on GCs belonging to the metal-poor range, [Fe/H]zw<-1.1 dex, obtained from observations with different equipment, but calibrated by standard stars situated in the observed cluster fields. We confirm our earlier finding, and extend it to a larger magnitude range.We demonstrate that DeltaU_{TO}^{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, Delta U_{TO}^{RGB}, 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 DeltaU_{TO}^{RGB} on [Fe/H] and have estimated its analytical expression, by assuming a linear relation between the parameter and metallicity. Its slope, Delta U_{TO}^{RGB}/Delta [Fe/H]~1.2 mag/dex, is approx. 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 M75 are also discussed.

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