Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm

• Published in: The Astrophysical journal Vol. 946; no. 1; pp. 48 - 79
• Main Authors: Hattori, Kohei, Okuno, Akifumi, Roederer, Ian U.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.03.2023; IOP Publishing
• Subjects: Astroinformatics; Astrophysics; Clustering; Dwarf galaxies; Galactic archaeology; Galactic halos; Greedy algorithms; Milky Way; Milky Way dynamics; Milky Way stellar halo; Orbits; R-process; Solar neighborhood; Star mergers; Stars; Stars & galaxies; Stellar dynamics; Stellar evolution
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/acb93b

Abstract 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.