Milky Way's Metal-Poor Stars display Chemical Transition near the Solar Radius

TL;DR

This study reveals a significant chemical transition in metal-poor stars near the Solar radius in the Milky Way, indicating different formation histories inside and outside this boundary.

Contribution

It identifies a chemical transition at 8 kpc in the Milky Way's metal-poor stars, suggesting distinct origins and enrichment processes for inner and outer populations.

Findings

Nine abundances show a transition near 8 kpc

Transition is unlikely due to radial migration

Inner and outer populations likely have different origins

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 $r_{\rm GC}$ vs. [Fe/H] and $r_{\rm GC}$ vs. [X/Fe] -- and if yes, then what is their implication for Galaxy formation. We combine the datasets of APOGEE DR17 and $\textit{Gaia}$ 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 ($|z|\geq1$ kpc), and explore a total of $19$ abundances. We find that $9$ different abundances exhibit a drastic transition in their distribution near the Solar radius $r_{\rm GC}=8$ 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 is a dominant metal-poor population of the Galaxy, and find that it 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.