毕竟不是一个疏散星团：Aquila的NGC 6863集群
Not an open cluster after all: the NGC 6863 asterism in Aquila
摘要：
出生后不久，疏散星团开始溶解，恒星逐渐消失在周围的恒星场中。完全解体的时间尺度既取决于它们的初始成员资格，也取决于它们在银河系中的位置。疏散星团经历了星团破裂的末期或疏散星团的残余物（OCR）是出了名的难以识别。从观测的角度来看，低数量的统计数据和与一般恒星场的最小对比度的结合，使它们成为非常具有挑战性的物体。使情况更糟的是，场星的随机样本经常显示出可能导致将它们错误地归类为极端进化的疏散星团的特征。在本文中，我们提供了一个详细的研究，恒星的内容和运动学的NGC 6863，一个紧凑的几颗恒星位于天鹰座和描述的POSS作为一个不存在的集群。尽管如此，这个物体最近被归类为OCR。本工作的目的是通过详细的逐星分析来确认或反驳其OCR状态。分析是使用宽视场测光在UBVI通带，自行从UCAC3目录，和高分辨率光谱，以及从广泛的$N$体计算的结果。我们的研究结果显示，NGC 6863的四颗最亮的恒星形成了一个星团，一组非物理关联的恒星投射在一起，从而得出NGC 6863不是一个真正的疏散星团的结论。
英文摘要：
Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters. In this paper, we provide a detailed study of the stellar content and kinematics of NGC 6863, a compact group of a few stars located in Aquila and described by the POSS as a non existent cluster. Nonetheless, this object has been recently classified as OCR. The aim of the present work is to either confirm or disprove its OCR status by a detailed star-by-star analysis. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC3 catalogue, and high resolution spectroscopy as well as results from extensive $N$-body calculations. Our results show that the four brightest stars commonly associated to NGC 6863 form an asterism, a group of non-physically associated stars projected together, leading to the conclusion that NGC 6863 is not a real open cluster.
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