The r-process enhancement of the metal-poor stars in the Magellanic Clouds

The chemical abundances of the most metal-poor stars in a galaxy can be used to investigate the earliest stages of its formation and chemical evolution. Differences between the abundances of the most metal-poor stars in the Milky Way and in its satellite dwarf galaxies have been noted and provide the strongest available constraints on the earliest stages galactic chemical evolution models. However, the masses of the Milky Way and its satellite dwarf galaxies differ by four orders of magnitude, leaving a gap in our knowledge of the early chemical evolution of intermediate mass galaxies like the Magellanic Clouds. To close the gap, we have extended the Best & Brightest survey to the Magellanic Clouds using the mid-infrared metal-poor star selection of Schlaufman & Casey (2014). We have discovered the most metal-poor giant stars known in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). The stars have metallicities in the range -2.6 < [Fe/H] < -2.0. We observed a strinkingly high level of r-process enhancement. Nine out of eleven stars in our sample are r-process (~81%). We estimate the probability of randomly selected very metal-poor stars in the halo of the Milky Way to have such high level of r-process enhancement to be as low as 1 in 10⁹. Therefore, the early chemical enrichment of the heaviest elements in the Magellanic Clouds and Milky Way were qualitatively different, possibly due to the different histories of interaction of the Milky Way and Magellanic Clouds with satellite galaxies.