The detailed chemical abundance patterns of accreted halo stars from the optical to infrared

TL;DR

This study investigates the detailed chemical abundance patterns of accreted halo stars from Gaia-Enceladus/Gaia-Sausage using optical and infrared spectra, revealing their unique nucleosynthesis signatures and star formation history.

Contribution

It provides a comprehensive comparison of optical and infrared abundance measurements for accreted halo stars, highlighting their neutron-capture element enhancements and lower alpha-element trends.

Findings

Optical and infrared abundances agree within 0.15 dex for most elements.

Accreted halo stars show neutron-capture element enhancements and r-process signatures.

These stars have lower alpha-to-iron ratios, indicating formation in a low star formation rate environment.

Abstract

Understanding the assembly of our Galaxy requires us to also characterize the systems that helped build it. In this work, we accomplish this by exploring the chemistry of accreted halo stars from the Gaia-Enceladus/Gaia-Sausage (GES) selected in the infrared from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16. We use high resolution optical spectra for 62 GES stars to measure abundances in 20 elements spanning the $\alpha$, Fe-peak, light, odd-Z, and notably, the neutron-capture groups of elements to understand their trends in the context of and in contrast to the Milky Way and other stellar populations. Using these derived abundances we find that the optical and the infrared abundances agree to within 0.15 dex except for O, Co, Na, Cu, and Ce. These stars have enhanced neutron-capture abundance trends compared to the Milky Way, and their [Eu/Mg] and neutron-capture abundance ratios (e.g., [Y/Eu], [Ba/Eu], [Zr/Ba], [La/Ba], and [Nd/Ba]) point to r-process enhancement and a delay in s-process enrichment. Their [$\alpha$/Fe] trend is lower than the Milky Way trend for [Fe/H]$>$-1.5 dex, similar to previous studies of GES stars and consistent with the picture that these stars formed in a system with a lower rate of star formation. This is further supported by their depleted abundances in Ni, Na, and Cu abundances, again, similar to previous studies of low-$\alpha$ stars with accreted origins.