Origin of highly r-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy. Issue 4 (14th November 2022)

• Record Type: Journal Article
• Authors: Hirai, Yutaka; Beers, Timothy C; Chiba, Masashi; Aoki, Wako; Shank, Derek; Saitoh, Takayuki R; Okamoto, Takashi; Makino, Junichiro
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 517:Issue 4(2022)
• Page Start: 4856
• Page End: 4874
• Publication Date: 2022-11-14
• Subjects: methods: numerical -- stars: abundances -- Galaxy: abundances -- Galaxy: formation -- Galaxy: halo -- Galaxy: kinematics and dynamics; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac2489
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: The r -process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way (MW) and the nucleosynthesis of the heaviest elements. Observations by the R -Process Alliance (RPA) and others have confirmed that many RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf galaxies of the MW. However, we do not know how RPE stars are formed. Here, we present the result of a cosmological zoom-in simulation of an MW-like galaxy with r -process enrichment, performed with the highest resolution in both time and mass. Thanks to this advancement, unlike previous simulations, we find that most highly RPE ( r -II; [Eu/Fe] > +0.7) stars are formed in low-mass dwarf galaxies that have been enriched in r -process elements for [Fe/H] $\lt -2.5$, while those with higher metallicity are formed in situ, in locally enhanced gas clumps that were not necessarily members of dwarf galaxies. This result suggests that low-mass accreted dwarf galaxies are the main formation site of r -II stars with [Fe/H] $\, \lt -2.5$ . We also find that most low-metallicity r -II stars exhibit halo-like kinematics. Some r -II stars formed in the same halo show low dispersions in [Fe/H] and somewhat larger dispersions of [Eu/Fe], similar to the observations. The fraction of simulated r -II stars is commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo r -II stars matches that observed. These results demonstrate that RPE stars can be valuable probes of the accretion of dwarf galaxies in the early stages of their formation.