M55中蓝离散星径向分布的令人惊讶的外部向上翻转
The surprising external upturn of the Blue Straggler radial distribution in M55
摘要：
通过结合高分辨率的HST和宽视场的地面观测，在紫外和光学波段，我们研究了低密度星系团球状星团M55（NGC 6809）在其整个径向范围内的蓝离散星（BSS）的人口。BSS预计的径向分布被发现是双峰，与一个中心峰，一个广泛的最小值在中间半径，和一个上翘在大半径。类似的双峰分布在其他球状星团（M3，47 Tuccott，NGC 6752，M5）中也被发现，但M55的外部上升是迄今为止发现的最大的。这可能表明在M55的外围区域有很大比例的原始双星，这似乎与最近在该星团核心测量到的相对较低（\sim 10%）的双星比例形成了对比。
英文摘要：
By combining high-resolution HST and wide-field ground based observations, in ultraviolet and optical bands, we study the Blue Straggler Star (BSS) population of the low density galactic globular cluster M55 (NGC 6809) over its entire radial extent. The BSS projected radial distribution is found to be bimodal, with a central peak, a broad minimum at intermediate radii, and an upturn at large radii. Similar bimodal distributions have been found in other globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in M55 is the largest found to date. This might indicate a large fraction of primordial binaries in the outer regions of M55, which seems somehow in contrast with the relatively low (\sim 10%) binary fraction recently measured in the core of this cluster.
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