Origin of highly r-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy

• DOI: 10.1093/mnras/stac2489
• Publication Date: 2022-11-14T05:08:50Z
• Authors: Yutaka Hirai; Timothy C Beers; Masashi Chiba; Wako Aoki; Derek Shank; Takayuki R Saitoh; Takashi Okamoto; Junichiro Makino
• Keywords: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics - Solar and Stellar Astrophysics; Astrophysics of Galaxies (astro-ph.GA); 0103 physical sciences; FOS: Physical sciences; Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Astrophysics of Galaxies; 01 natural sciences; Solar and Stellar Astrophysics (astro-ph.SR)

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.