一袋技巧：利用银河系恒星的自行来识别武仙座超微弱的矮星系成员
A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members
摘要：
武仙座是超暗矮星系（UFD）的原型。到目前为止，由于很难将武仙座真正的恒星从严重的银河场污染中分离出来，因此对它的总光度仍然没有严格的限制。为了更好地约束武仙座属性，我们的目标是使用银河系恒星的自行和颜色-颜色图来去除星系场中的前景和背景污染物。我们已经获得了大力士的图像在rSloan，BBessel和Uspec波段与大双筒望远镜（LBT）和LBC-BIN模式的能力。rSloan的新数据集与LBT档案中的数据相结合，跨越了大约5年的时间基线，使我们能够首次测量武仙座方向的恒星自行。Uspec数据与现有的LBT光度测量一起使我们能够使用颜色-颜色图来进一步消除现场污染。由于采用了一种高精度的方法来推导出LBC-red的rSloan滤波器几何畸变解，我们能够测量恒星的相对自行，其精度优于5 mas yr^-1，一直到rSloan=22等，并且能够分离出相当大一部分（> 90%）的银河系污染物。我们最终得到了528个源的样本，分布在星系体的很大一部分（0.12 deg^2）。在这些来源中，有171个是背景星系和额外的前景恒星，从分析Uspec - BBessel与BBessel - rSloan的色-色图。这给我们留下了357个可能的武仙座UFD成员的样本。我们将清洁的颜色星等图（CMD）与进化模型和合成CMD进行了比较，证实了武仙座中存在一个古老的种群（t=12 \pm 2 Gyr），金属丰度分布广泛（-3.3\<[Fe/H]\<-1.8）。
英文摘要：
Hercules is the prototype of the ultra-faint dwarf (UFD) galaxies. To date, there are still no firm constraints on its total luminosity, due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. In order to better constrain Hercules properties we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. We have obtained images of Hercules in the rSloan, BBessel and Uspec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The rSloan new data-set combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure for the first time proper motions of stars in the Hercules direction. The Uspec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination. Thanks to a highly-accurate procedure to derive the rSloan-filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5 mas yr^-1 down to rSloan=22 mag and disentangle a significant fraction (\>90\%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (0.12 deg^2). Of these sources, 171 turned out to be background galaxies and additional foreground stars, from the analysis of the Uspec - BBessel vs. BBessel - rSloan colour-colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of an old population (t=12\pm 2 Gyr), with a wide spread in metallicity (-3.3\<[Fe/H]\<-1.8).
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