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The r-Process in the Milky Way and Beyond : r-Process-Enhanced Metal-Poor Stars and Globular Clusters

Charli Sakari

The elements that form via the rapid (r-) neutron-capture process are valuable tracers of a galaxy’s assembly history. Highly r-process-enhanced metal-poor stars have been discovered in the Milky Way (MW) halo, in globular clusters (GCs), and in satellite ultra-faint dwarf galaxies. These stars show significant enhancements in elements like Eu and Ba, despite their deficiencies in Fe ; they have therefore retained the distinct chemical signature of an r-process event. Although the recent detection of GW 170817 has shown that r-process nucleosynthesis occurs in neutron star mergers, it remains uncertain if other objects (e.g., supernovae) can also create r-process elements. The population of r-process-enhanced stars will be crucial for identifying the site(s), environments, and occurrence rates of r-process nucleosynthesis across cosmic time and in various environments. In this talk, I will present the first results from the the R-Process Alliance (RPA), a collaboration with the goal of identifying the site(s) of the r-process. The first major task of the RPA is an observational campaign to identify new r-process-enhanced metal-poor stars in the MW. So far, the RPA has discovered >100 r-process-enhanced metal-poor stars, nearly doubling the known number. The combination of detailed abundances with kinematic results from Gaia means that the origins of these stars can be explored, leading to important implications for the assembly history of the MW halo. I will also briefly discuss r-process-enhancement in GCs, both in the MW and M31. Stars in distant GCs are more difficult to observe, due to the faintness of the stars ; I will discuss results from high-resolution integrated light spectroscopy of entire GCs in M31, and the implications for M31’s assembly history.