高R-过程增强场星的动力学：吸积起源的证据和来自破裂超新星的几个星群的探测
Kinematics of Highly R-Process-Enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites
摘要：
我们给出了35颗高r过程增强（[Eu/Fe] >= +0.7）贫金属（-3.8 < [Fe/H] < -1.4）场星的运动学。我们计算六维的位置和速度，评估能量和运动积分，并使用第二次盖亚数据发布和公布的径向速度中的惯性和自行来计算这些恒星的轨道。所有这些恒星都有晕轮运动学。大多数恒星（66%）仍然处于晕的内部区域（< 13 kpc），许多（51%）的轨道在银河系中心的2.6 kpc范围内。有几颗恒星（20%）的轨道延伸超过20 kpc，其中一颗的远心轨道大于银河系维里半径。我们应用三种聚类方法来搜索相空间中的结构，我们确定了八组。没有丰度被认为是在聚类过程中，但[Fe/H]分散的群体是小于预期的随机机会。轨道特性，在相空间和金属丰度的集群，以及缺乏高度r-过程增强的恒星在盘状轨道上表明，这些恒星可能是从被破坏的卫星吸积。与星系光度-金属丰度关系的比较表明，大部分的祖先卫星的M_V > -9，是超暗或低光度的经典矮椭球星系的特征。低速率的恒星形成和铁生产的环境，而不是r-过程网站的性质，可能是关键，以获得[Eu/Fe]比率发现在高度r-过程增强的恒星。
英文摘要：
We present kinematics of 35 highly r-process-enhanced ([Eu/Fe] >= +0.7) metal-poor (-3.8 < [Fe/H] < -1.4) field stars. We calculate six-dimensional positions and velocities, evaluate energies and integrals of motion, and compute orbits for each of these stars using parallaxes and proper motions from the second Gaia data release and published radial velocities. All of these stars have halo kinematics. Most stars (66%) remain in the inner regions of the halo (< 13 kpc), and many (51%) have orbits that pass within 2.6 kpc of the Galactic center. Several stars (20%) have orbits that extend beyond 20 kpc, including one with an orbital apocenter larger than the Milky Way virial radius. We apply three clustering methods to search for structure in phase space, and we identify eight groups. No abundances are considered in the clustering process, but the [Fe/H] dispersions of the groups are smaller than would be expected by random chance. The orbital properties, clustering in phase space and metallicity, and lack of highly r-process-enhanced stars on disk-like orbits indicate that such stars likely were accreted from disrupted satellites. Comparison with the galaxy luminosity-metallicity relation suggests M_V > -9 for most of the progenitor satellites, characteristic of ultra-faint or low-luminosity classical dwarf spheroidal galaxies. Environments with low rates of star formation and Fe production, rather than the nature of the r-process site, may be key to obtaining the [Eu/Fe] ratios found in highly r-process-enhanced stars.
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