Kaley Brauer, Massachusetts Institute of Technology

Photo of Kaley Brauer

The motions and chemical composition of stars present in the extended outskirts (called the stellar halo) of the Milky Way preserve a record of the galaxy's formation history. Galaxies form hierarchically by absorbing smaller galaxies to grow larger. While most of the stars in the center and disk of a galaxy formed in situ, many of those in the stellar halo originated in the many small galaxies that the central host galaxy accreted over billions of years. Currently, however, we lack reliable ways to identify which halo stars originated in which dwarf galaxies or even which stars were definitively accreted. Selecting stars with specific chemical signatures may provide a way forward. We investigate this theoretically and observationally for stars with r-process nucleosynthesis signatures. Theoretically, we combine high-resolution cosmological simulations with an empirically motivated treatment of r-process enhancement. We find that around half of highly r-process-enhanced metal-poor halo stars may have originated in early ultra-faint dwarf galaxies that merged into the Milky Way during its formation. Observationally, we use Gaia DR2 to compare the kinematics of highly r-process-enhanced halo stars with those of normal halo stars. R-process-enhanced stars have higher galactocentric velocities than normal halo stars, suggesting an accretion origin. If r-process-enhanced stars largely originated in accreted ultra-faint dwarf galaxies, halo stars we observe today could play a key role in understanding the smallest building blocks of the Milky Way via this novel approach of chemical tagging.

Abstract Author(s): Kaley Brauer, Alexander P. Ji, Sergio Escobar, Anna Frebel