The chemical imprint of the bursty nature of Milky Way's progenitors

Carbon-enhanced metal-poor (CEMP) stars with low abundances of neutron capture elements (CEMP-no stars) are ubiquitous among metal-poor stars in the Milky Way. Recent observations have uncovered their two subgroups that differ in the carbon to magnesium ([C/Mg]) abundance ratio. Here, we demonstrate that similar abundance patterns are also present in Milky Way-like galaxies in the EAGLE cosmological hydrodynamical simulation, where these patterns originate from the fact that stars may form from gas enriched predominantly by asymptotic giant branch (AGB) stars or by type II supernovae. This occurs when stars form in the poorly mixed interstellar medium of Milky Way progenitor galaxies that are undergoing bursty star formation. The CEMP-no stars with lower [C/Mg] form at the onset of a starburst from gas enriched by low-metallicity type II supernovae that power a strong outflow, quenching further star formation. When star formation resumes following cosmological gas accretion, the CEMP-no stars with higher [C/Mg] form, with enrichment by AGB ejecta evident in their higher abundance of barium and lower abundance of magnesium. This suggests that bursty star formation in the progenitors of the Galaxy leaves a permanent imprint in the abundance patterns of CEMP stars.