Milky Way’s Metal-poor Stars Display Chemical Transition near the Solar Radius

Abstract The metal-poor stars of a galaxy offer insights into that galaxy’s early formation processes and accretion history. Here, we investigate whether the metal-poor stars of our Milky Way Galaxy exhibit any characteristic trends in Galactocentric distance versus chemical abundances—i.e., in the space of r GC versus [Fe/H] and r GC versus [X/Fe]—and if yes, then what their implication is for Galaxy formation. We combine the data sets of APOGEE DR17 and Gaia DR3, where the former provides stellar abundances and the latter provides stellar parallaxes. We analyze bright ( G < 13) and metal-poor ([Fe/H] < −1.2) stars located far from the disk (∣ z ∣ ≥ 1 kpc) and explore a total of 19 abundances. We find that nine different abundances exhibit a drastic transition in their distribution near the solar radius r GC = 8 kpc. This trend is very unlikely to be related to radial migration, as our metal-poor sample does not contain any disk star. We also analyze the Gaia-Sausage/Enceladus stars, which form a dominant metal-poor population of the Galaxy, and find that they alone cannot account for this trend. This suggests that the Milky Way’s metal-poor populations inside and outside the solar radius likely originated from distinct chemical enrichment scenarios and formation processes.