The ratio of [Eu/α] differentiates accreted/in situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters
Date Issued
2024
Author(s)
Monty, Stephanie
•
Belokurov, Vasily
•
Sanders, Jason L
•
Hansen, Terese T
•
Sakari, Charli M
•
McKenzie, Madeleine
•
Myeong, GyuChul
•
Davies, Elliot Y
•
Ardern-Arentsen, Anke
•
Abstract
We combine stellar orbits with the abundances of the heavy, 𝑟-process element europium and the light, 𝛼-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/𝛼] 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 54 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.
Volume
533
Issue
2
Start page
2420
Issn Identifier
0035-8711
Rights
open.access
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