The ratio of [Eu/$\alpha$] differentiates accreted/in-situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

TL;DR

This study uses europium and silicon abundances combined with stellar orbits to distinguish in-situ and accreted stars in the Milky Way, revealing systematic differences in [Eu/Si] across metallicities and galaxy components.

Contribution

It introduces [Eu/Si] as a chemical tag to differentiate stellar populations and links globular clusters to their host galaxy's star formation history.

Findings

Accreted halo stars show higher [Eu/Si] at high energies.

In-situ stars have lower [Eu/Si], especially at lower energies.

Globular clusters' [Eu/Si] reflects their host galaxy's evolution.

Abstract

We combine stellar orbits with the abundances of the heavy, $r$-process element europium and the light, $\alpha$-element, silicon to separate in-situ and accreted populations in the Milky Way across all metallicities. At high orbital energy, the accretion-dominated halo shows elevated values of [Eu/Si], while at lower energies, where many of the stars were born in-situ, the levels of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the MW and the accreted halo imply that the scatter in [Eu/$\alpha$] within a single galaxy is smaller than previously thought. At the lowest metallicities, we find that both accreted and in-situ populations trend down in [Eu/Si], consistent with enrichment via neutron star mergers. Through compiling a large dataset of abundances for 46 globular clusters (GCs), we show that differences in [Eu/Si] extend to populations of in-situ/accreted GCs. We interpret this consistency as evidence that in $r$-process elements, GCs trace the star formation history of their hosts, motivating their use as sub-Gyr timers of galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple galactic chemical evolution model, we find that differences in [Eu/Si] between accreted and in-situ MW field stars cannot be explained through star formation efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag between GCs and their host galaxies extends beyond the Local Group, to the halo of M31 - potentially offering the opportunity to do Galactic Archaeology in an external galaxy.