Selecting accreted populations: metallicity, elemental abundances, and ages of the Gaia-Sausage-Enceladus and Sequoia populations

TL;DR

This study uses Gaia kinematics and APOGEE elemental abundances to identify and characterize the metallicity, elemental patterns, ages, and progenitor masses of the Gaia-Sausage-Enceladus and Sequoia accretion populations in the Milky Way.

Contribution

It provides a detailed chemical and kinematic analysis of GSE and Sequoia populations, improving selection methods and understanding their properties.

Findings

GSE has mean [Fe/H] ~ -1.15 and ages 10-12 Gyr.

Sequoia has mean [Fe/H] ~ -1.3 and ages 12-14 Gyr.

Sequoia shows a complex elemental abundance distribution.

Abstract

Identifying stars found in the Milky Way as having formed in situ or accreted can be a complex and uncertain undertaking. We use Gaia kinematics and APOGEE elemental abundances to select stars belonging to the Gaia-Sausage-Enceladus (GSE) and Sequoia accretion events. These samples are used to characterize the GSE and Sequoia population metallicity distribution functions, elemental abundance patterns, age distributions, and progenitor masses. We find that the GSE population has a mean [Fe/H] $\sim -1.15$ and a mean age of $10-12$ Gyr. GSE has a single sequence in [Mg/Fe] vs [Fe/H] consistent with the onset of SN Ia Fe contributions and uniformly low [Al/Fe] of $\sim -0.25$ dex. The derived properties of the Sequoia population are strongly dependent on the kinematic selection. We argue the selection with the least contamination is $J_{\phi}/J_{\mbox{tot}} < -0.6$ and $(J_z - J_R)/J_{\mbox{tot}} < 0.1$. This results in a mean [Fe/H] $\sim -1.3$ and a mean age of $12-14$ Gyr. The Sequoia population has a complex elemental abundance distribution with mainly high [Mg/Fe] stars. We use the GSE [Al/Fe] vs [Mg/H] abundance distribution to inform a chemically-based selection of accreted stars, which is used to remove possible contaminant stars from the GSE and Sequoia samples.