R-Process Alliance

Near Field Cosmology

The R-Process Alliance (RPA) is a collaboration started in 2016, including astronomers, modelers, nuclear theorists, and experimentalists. The RPA seeks to answer the many questions about the rapid neutron-capture process responsible for producing roughly half of the heavy elements beyond iron in the Solar System. The first phase of the RPA focuses on searching for r-process enhanced stars among bright metal-poor stars in the Milky Way halo (DR1, DR2, DR3, DR4).

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Ultra faint dwarf (UFD) galaxies are the smallest, most dark matter dominated, and metal-poor systems we know. Detailed chemical analyses of stars in UFD galaxies provide a window to study stars' birth environment and examine single nucleosynthesis enrichment events. The UFD galaxies' small closed environments also provide additional physical constraints on the type of events that can have enriched the system and the mixing of the ejecta into the interstellar medium.

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[Mg/Ca] ratios of stars in ultra-faint dwarf galaxies. The three stars in Grus II all exhibit high [Mg/Ca] ratios suggesting this galaxy was enriched by a population of high mass stars (Hansen et al. 2020).

©2020 by Terese Hansen

Galactic Archaeology

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The chemical composition of the oldest and most metal-poor stars in the Milky Way is a direct fingerprint of the elements produced in the stellar generations before them. Therefore, they are ideal tools to study specific nucleosynthesis channels.

Absolute abundances of Sr and Ba, as a function of [Fe/H] for CEMP stars. This data suggests a floor exists for Barium abundances of CEMP-no stars shown with red symbols (Hansen et al. 2015).