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