Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm
Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm
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DOI：
10.3847/1538-4357/acb93b
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发表时间：
2022-07
期刊：
影响因子：
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通讯作者：
Kohei Hattori;Akifumi Okuno;I. Roederer
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文献类型：
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作者：
Kohei Hattori;Akifumi Okuno;I. Roederer
具有[Eu/Fe]≥+0.7R过程增强星(所谓的r-II星)被认为是在极富中子的环境中形成的，在这种环境中发生了一种罕见的天体物理事件(例如，中子星合并)。这一情景得到了超暗矮星系网状星系II的存在的支持，在该星系中，大多数恒星的r过程元素都得到了高度增强。在这种情况下，银河系周围的一些矮小星系被增强了。当每个r-增强型矮星系聚集到银河系时，它在银晕中沉积了许多具有类似轨道行为的r-II恒星。为了寻找r增强系统的残留物，我们利用Gaia EDR3数据分析了N=161颗r-II星在太阳附近的轨道行为分布。由于观测的不确定性不可忽视，我们将一种新开发的贪婪乐观聚类法应用于我们的样本恒星的轨道行为。我们发现了六个具有相似轨道和化学成分的r-II星团，其中一个是新发现的。考虑到成员恒星的明显相混合轨道，我们解释说，这些星团是与古老银河系合并的完全分裂的r增强矮星系残骸的很好候选者。
R-process enhanced stars with [Eu/Fe] ≥ +0.7 (so-called r-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a neutron-star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum II, where most of the stars are highly enhanced in r-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way were r enhanced. When each r-enhanced dwarf galaxy accreted to the Milky Way, it deposited many r-II stars in the Galactic halo with similar orbital actions. To search for the remnants of the r-enhanced systems, we analyzed the distribution of the orbital actions of N = 161 r-II stars in the solar neighborhood by using Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly developed greedy optimistic clustering method to the orbital actions of our sample stars. We found six clusters of r-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disrupted r-enhanced dwarf galaxies that merged with the ancient Milky Way.