Presentation #216.02 in the session The Milky Way: Structure, Mass Distribution, and Stellar Populations.
R-process-enhanced (RPE) stars are fossil records of the nucleosynthesis and assembly history of the Milky Way. Ongoing efforts of the R-Process Alliance have confirmed hundreds of RPE stars in the Galaxy. Together with Gaia data, many of the RPE stars are found to be associated with chemo-dynamically tagged groups (CDTGs), which could come from accreted dwarf galaxies. However, we still do not know when and where such stars are formed. We have performed a series of high-resolution cosmological zoom-in simulations of Milky Way-like galaxies with a gas-mass resolution of 10000 solar masses, including r-process enrichment by neutron star mergers. Here we report that accreted dwarf galaxies are the main formation site of RPE stars with [Fe/H] < -2. Most RPE stars show halo-like kinematics, different from the Solar Neighborhood stars. This result is consistent with the R-Process Alliance observations. We found that over 90% of RPE stars are formed at redshifts higher than z = 1.6. There are increasing trends in the fraction of RPE stars formed in accreted components toward higher [Eu/Fe] and lower [Fe/H]. We also found that RPE stars formed in the same accreted halos exhibit a scatter of [Eu/Fe] ratios and decreasing trend of [Mg/Fe] ratios toward higher metallicity. Our simulations suggest that RPE stars are more easily formed in subclumps than in the main progenitor halo because of subclumps’ small gas masses and lower star-formation efficiencies. This scenario supports that neutron star mergers are a dominant site of the r-process. These results demonstrate that RPE stars can be useful probes of the early stages of Milky Way formation.
This work was supported in part from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation.