Origin of Highly $r$-Process-Enhanced Stars in a Cosmological Zoom-in Simulation of a Milky Way-like Galaxy

The $r$-process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way and the nucleosynthesis of the heaviest elements. Analysis of observations by the $R$-Process Alliance (RPA) and others have confirmed that hundreds of RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf satellite galaxies of the Milky Way (MW). However, we still do not know how RPE stars are formed in the MW. Here, we perform a high-resolution cosmological zoom-in simulation of an MW-like galaxy and demonstrate that RPE stars are primarily formed in gas clumps enhanced in $r$-process elements. For [Fe/H] $\,<-2.5$, 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, 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] $\,<-2.5$. 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], as seen in the observations. The fraction of $r$-II stars found in our simulation is also commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo $r$-II stars well matches that observed. These results demonstrate that RPE stars can also be valuable probes of the early stages of their formation.