Main-sequence Turnoff Stars as Probes of the Ancient Galactic Relic: Chemo-dynamical Analysis of a Pilot Sample

Abstract The main-sequence turnoff (MSTO) stars well preserve the chemical properties where they were born, making them ideal tracers for studying the stellar population. We perform a detailed chemo-dynamical analysis on moderately metal-poor (−2.0 < [Fe/H] < −1.0) MSTO stars to explore the early accretion history of the Milky Way. Our sample includes four stars observed with high-resolution spectroscopy using ESPaDOnS at the Canada–France–Hawaii Telescope and 163 nearby MSTO stars selected from the SAGA database with high-resolution results. Within the action-angle spaces, we identified Gaia–Sausage–Enceladus (GSE, 35 objects), stars born in the Milky Way (in situ, 31 objects), and other substructures (21 objects). We find that both GSE and in situ stars present a similar Li plateau around A (Li) ∼ 2.17. GSE shows a clear α -knee feature in Mg at [Fe/H] ∼ −1.60 ± 0.06, while the α -elements of in situ stars remain nearly constant within this metallicity range. The iron-peak elements show little difference between GSE and in situ stars except for Zn and Ni, which decrease in GSE at [Fe/H] > −1.6, while they remain constant for in situ stars. Among heavy elements, GSE shows overall enhancement in Eu, with [Ba/Eu] increasing with the metallicity, while this ratio remains almost constant for in situ stars, suggesting the contribution of longer timescale sources to the s -process in GSE. Moreover, for the first time, we present the r -process abundance pattern for an extremely r -process enhanced ( r -II) GSE star, which appears consistent with the solar r -process pattern except for Pr. Further investigation of larger GSE samples using high-resolution spectra is required to explore the reason for the significantly higher Pr in the GSE r -II star.