UB CCD PHOTOMETRY OF THE OLD, METAL-RICH, OPEN CLUSTERS NGC 6791, NGC 6819, AND NGC 7142
UB CCD PHOTOMETRY OF THE OLD, METAL-RICH, OPEN CLUSTERS NGC 6791, NGC 6819, AND NGC 7142
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古老、富含金属、疏散星团 NGC 6791、NGC 6819 和 NGC 7142 的 UB CCD 光度测量
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发表时间：
2013
期刊：
影响因子：
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通讯作者：
L. Buson
中科院分区：
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作者：
G. Carraro;A. Buzzoni;E. Bertone;L. Buson
我们报告了对古老的、富含金属的疏散星团NGC 6791、NGC 6819和NGC 7142的紫外定向成像调查。这三个星团具有超太阳的金属丰度和年龄在3-8 Gyr以上，代表了非常接近和理想的恒星聚集，以匹配演化恒星群的独特特性，如椭圆星系和螺旋星系的凸起。在对NGC 6791观测的第一次讨论之后，我们在这里完成了我们的分析，同时也对NGC 6819和NGC 7142进行了首次U CCD光度测量。详细分析了三个星团的色星等图，特别强调了热恒星成分。在这方面，我们报告了ngc6791中一颗新的极端水平分支候选星。对于NGC 6819和7142来说，恒星光度函数清楚地指向了暗淡的较低主序星的松散径向分布，这可能是星团与星系动态相互作用的结果，也可能是越来越多的双星朝向星团核心的影响，就像在NGC 6791中观察到的那样。与星系盘的参考理论模型相比，沿着每个星团视线的恒星场分析表明，一个更集中的厚盘，在短于2.8 kpc的尺度上，可能更好地调和观测到的较低比例的亮场恒星及其白矮星后代。
We report on a UV-oriented imaging survey in the fields of the old, metal-rich open clusters NGC 6791, NGC 6819, and NGC 7142. With their super-solar metallicity and ages ≳ 3–8 Gyr, these three clusters represent both very near and ideal stellar aggregates to match the distinctive properties of the evolved stellar populations, as in elliptical galaxies and bulges of spirals. Following a first discussion of NGC 6791 observations in an accompanying paper, here we complete our analysis, also presenting for NGC 6819 and NGC 7142 the first-ever U CCD photometry. The color–magnitude diagram of the three clusters is analyzed in detail, with special emphasis on the hot stellar component. We report, in this regard, one new extreme horizontal-branch star candidate in NGC 6791. For NGC 6819 and 7142, the stellar luminosity function clearly points to a looser radial distribution of faint lower main sequence stars, either as a consequence of cluster dynamical interaction with the Galaxy or as an effect of an increasing fraction of binary stars toward the cluster core, as also observed in NGC 6791. Compared to a reference theoretical model for the Galaxy disk, the analysis of the stellar field along the line of sight of each cluster indicates that a more centrally concentrated thick disk, on a scale length shorter than ∼2.8 kpc, might better reconcile the lower observed fraction of bright field stars and their white-dwarf progeny.