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Space Velocities of Globular Clusters. III. Cluster Orbits and Halo Substructure

机译:球状星团的空间速度。三,团簇轨道和晕子结构

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We have compiled a catalog of absolute proper motions of globular clusters from various sources. The sample consists of 38 clusters, from which most of the southern ones (15 clusters) were measured in our previous papers in this series. We have integrated orbits assuming two different Galactic potential models adopted from the literature and have calculated orbital parameters. The uncertainties associated with the orbital parameters were derived in a Monte Carlo approach, and we conclude that, overall, at the present level of measurement errors, orbital differences due to Galactic potential models are not significant. Three metal-poor clusters are found to have orbits similar to prototypical metal-rich disk clusters. These clusters are NGC 6254 (M10), NGC 6626 (M28), and NGC 6752. We interpret this as a potentially significant constraint on the formation of the disk. It is thus possible that part of the inner metal-poor halo is the low-metallicity tail of the thick disk. In this case, the ages of these clusters indicate that the formation of the disk partially overlapped with that of the halo. The clusters classified as "young halo" or "red horizontal-branch" by Zinn show a radially anisotropic velocity distribution, their orbits are of high total energy, with apocentric radii larger than 10 kpc and highly eccentric. In this sense they may represent an accreted component of our Galaxy. We also discuss ω Cen's orbit characteristics in the view of an accreted origin. We investigate the effect of the orbital motion on the internal dynamics of clusters. Adopting the formalism from Gnedin & Ostriker and their destruction rates due to two-body relaxation, we find that, in most cases, this internal process is more important than the destruction processes due to disk and bulge shocking. Hubble Space Telescope (HST) observations argue that NGC 6397's luminosity function is depleted at the faint end, and this is blamed on its high total destruction rate. We propose a list of clusters with similar destruction rates that may also have depleted luminosity functions. We also note the bias toward deriving higher destruction rates in studies that statistically assign tangential velocities based on a kinematic model of the globular cluster system, in contrast to the rates derived from the measured tangential velocities. Clusters prone to such biases are those that have circular orbits (kinematically thick-disk clusters) and some of those with orbits of high total energy.
机译:我们已经从各种来源收集了球状星团绝对正确运动的目录。该样本由38个聚类组成,在我们之前的系列文章中,从中测量了大多数南部聚类(15个聚类)。我们假设从文献中采用两种不同的银河势模型来对轨道进行积分,并计算出轨道参数。与轨道参数相关的不确定性是通过蒙特卡洛方法得出的,我们得出的结论是,总体而言,在目前的测量误差水平上,由于银河势模型引起的轨道差异并不显着。发现三个贫金属集群的轨道类似于典型的富金属磁盘集群。这些簇是NGC 6254(M10),NGC 6626(M28)和NGC6752。我们将其解释为对磁盘形成的潜在重要限制。因此,内部贫金属光环的一部分可能是厚盘的低金属性尾部。在这种情况下,这些簇的年龄表明圆盘的形成与晕圈的形成部分重叠。 Zinn分类为“年轻晕”或“红色水平分支”的星团表现出径向各向异性的速度分布,其轨道具有较高的总能量,其远心半径大于10 kpc,且偏心率很高。从这个意义上讲,它们可能代表了我们银河系的一个重要组成部分。我们还将根据增生原点讨论ωCen的轨道特性。我们研究了轨道运动对星团内部动力学的影响。我们采用Gnedin&Ostriker的形式主义以及由于两体松弛而造成的破坏率,我们发现,在大多数情况下,这种内部过程比由于磁盘和凸起冲击所引起的破坏过程更为重要。哈勃太空望远镜(HST)的观测结果表明,NGC 6397的光度功能在微弱的一端被消耗掉了,这归因于其较高的总破坏率。我们提出了具有相似破坏率的簇的列表,这些簇也可能具有耗尽的光度函数。我们还注意到,在基于球状聚类系统的运动学模型统计地分配切向速度的研究中,与从测得的切向速度得出的比率相比,偏向于推导更高的破坏率。容易产生这种偏差的星团是那些具有圆形轨道(运动学上较厚的盘状星团)的轨道,有些轨道具有较高的总能量。

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