Elevated r-process enrichment in Gaia Sausage and Sequoia
  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.
