Elevated r-process enrichment in Gaia Sausage and Sequoia

TL;DR

This study reveals significant r-process element enrichment in the Gaia Sausage and Sequoia stellar populations, providing insights into early Galactic assembly and neutron-capture element evolution.

Contribution

It presents high-resolution spectroscopic analysis of Gaia Sausage and Sequoia stars, highlighting r-process dominance and multiple s-process enrichment sequences in these accreted structures.

Findings

Detected high Eu abundance indicating strong r-process contribution.

Identified a transition from massive star to AGB star influence in element ratios.

Discovered two distinct s-process enrichment sequences in the stellar populations.

Abstract

The Gaia Sausage (GS) and the Sequoia represent the major accretion events that formed the stellar halo of the Milky Way. A detailed chemical study of these main building blocks provides a pristine view of the early steps of the Galaxy's assembly. We present the results of the analysis of the UVES high-resolution spectroscopic observations at the 8.2m VLT of 9 Sausage/Sequoia members selected kinematically using Gaia DR2. We season this set of measurements with archival data from Nissen & Schuster (2011) and GALAH DR3 (2020). Here, we focus on the neutron-capture process by analysing Sr, Y, Ba and Eu behavior. We detect clear enhancement in Eu abundance ([Eu/Fe]~0.6-0.7) indicative of large prevalence of r-process in the stellar n-capture makeup. We are also able to trace the evolution of the heavy element production across a wide range of metallicity. The barium to europium changes from a tight, flat sequence with [Ba/Eu]=-0.7 reflecting dominant contribution from exploding massive stars, to a clear upturn at higher iron abundances, betraying the onset of contamination from asymptotic giant branch (AGB) ejecta. Additionally, we discover two clear sequences in [Fe/H]-[Ba/Fe] plane likely caused by distinct levels of s-process pollution and mixing within the GS progenitor.