Age-Dating Red Giant Stars Associated with Galactic Disk and Halo
  Substructures
  The vast majority of Milky Way stellar halo stars were likely accreted from a
small number ($\lesssim$3) of relatively large dwarf galaxy accretion events.
However, the timing of these events is poorly constrained, relying
predominantly on indirect dynamical mixing arguments or imprecise age
measurements of stars associated with debris structures. Here, we aim to infer
robust stellar ages for stars associated with galactic substructures to more
directly constrain the merger history of the Galaxy. By combining kinematic,
asteroseismic, and spectroscopic data where available, we infer stellar ages
for a sample of 10 red giant stars that were kinematically selected to be
associated with the stellar halo, a subset of which are associated with the
Gaia-Enceladus-Sausage halo substructure, and compare their ages to 3 red giant
stars in the Galactic disk. Despite systematic differences in both absolute and
relative ages determined by this work, age rankings of stars in this sample are
robust. Passing the same observable inputs to multiple stellar age
determination packages, we measure a weighted average age for the
Gaia-Enceladus-Sausage stars in our sample of 8 $\pm$ 3 (stat.) $\pm$ 1 (sys.)
Gyr. We also determine hierarchical ages for the populations of
Gaia-Enceladus-Sausage, in situ halo and disk stars, finding a
Gaia-Enceladus-Sausage population age of 8.0$^{+3.2}_{-2.3}$ Gyr. Although we
cannot distinguish hierarchical population ages of halo or disk structures with
our limited data and sample of stars, this framework should allow distinct
characterization of Galactic substructures using larger stellar samples and
additional data available in the near future.
