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Shape parameters of Galactic open clusters

机译：银河系开放星团的形状参数

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Context. Ellipticities have been determined for only a few tens of open clusters. Aims. We derive the observed and modelled shape parameters (apparent ellipticity and orientation of the ellipse) of 650 Galactic open clusters identified in the ASCC-2.5 catalogue. Methods. We compute the observed shape parameters of Galactic open clusters with a multi-component analysis. For the vast majority of clusters, these parameters are determined for the first time. High resolution (``star by star'') N-body simulations are carried out with a specially developed GRAPE code providing models of clusters of different initial masses, Galactocentric distances, and rotation velocities. Results. By comparing models and observations for about 150 clusters, we find that the ellipticities of observed clusters are too low (0.2 vs. 0.3), and take a first step in identifying the main reason for this discrepancy. After 50Myr, the models predict that clusters exhibit an oblate shape with an axis ratio of 1.65:1.35:1, and a major axis tilt by an angle of with respect to the Galactocentric radius due to the differential rotation of the Galaxy. Conclusions. Unbiased estimates of cluster shape parameters require reliable membership determination in large cluster areas out to 2-3tidal radii, where the density of cluster stars is considerably lower than the background. Although dynamically bound stars beyond the tidal radius contribute insignificantly to the cluster mass, knowledge of their distribution is essential for a correct determination of cluster shape parameters. In contrast, a restricted mass range of cluster stars does not play such a dramatic role, although deep surveys allow us to identify more cluster members and, therefore, to increase the accuracy of the observed shape parameters. Key words: Galaxy: open clusters and associations: general - Galaxy:solar neighbourhood -Galaxy: stellar content
机译：上下文。仅对几十个开放星团确定了椭圆率。目的我们推导了在ASCC-2.5目录中确定的650个银河开放星团的观察和建模的形状参数（表观椭圆率和椭圆的方向）。方法。我们通过多分量分析计算观测到的银河系开放星团的形状参数。对于绝大多数群集，这些参数都是首次确定的。使用专门开发的GRAPE代码进行高分辨率（``逐个星星''）的N体模拟，该代码提供了不同初始质量，星心距离和旋转速度的星团模型。结果。通过比较大约150个星团的模型和观测值，我们发现观测到的星团的椭圆率太低（0.2 vs. 0.3），并迈出了确定这种差异的主要原因。在50Myr之后，模型预测星团呈扁圆形，轴比为1.65：1.35：1，并且由于星系的旋转差，主轴相对于半圆心半径倾斜了一个角度。结论。对星团形状参数的无偏估计需要在潮汐半径为2-3的大星团区域中进行可靠的隶属度确定，在该区域中，星团星的密度大大低于背景。尽管超过潮汐半径的动态束缚恒星对星团质量的贡献微不足道，但了解它们的分布对于正确确定星团形状参数至关重要。相反，尽管深入的调查使我们能够识别出更多的星团成员，从而提高了观测到的形状参数的准确性，但有限的星团质量范围并没有起到如此显着的作用。关键词：银河系：开放的集群和协会：一般-银河系：太阳能邻里-银河系：恒星含量

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