Identifying Sagittarius Stream Stars By Their APOGEE Chemical Abundance
  Signatures
  The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE)
survey provides precise chemical abundances of 18 chemical elements for $\sim$
176,000 red giant stars distributed over much of the Milky Way Galaxy (MW), and
includes observations of the core of the Sagittarius dwarf spheroidal galaxy
(Sgr). The APOGEE chemical abundance patterns of Sgr have revealed that it is
chemically distinct from the MW in most chemical elements. We employ a
\emph{k}-means clustering algorithm to 6-dimensional chemical space defined by
[(C+N)/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe] to identify 62 MW
stars in the APOGEE sample that have Sgr-like chemical abundances. Of the 62
stars, 35 have \emph{Gaia} kinematics and positions consistent with those
predicted by \emph{N}-body simulations of the Sgr stream, and are likely stars
that have been stripped from Sgr during the last two pericenter passages ($<$ 2
Gyr ago). Another 20 of the 62 stars exhibit chemical abundances
indistinguishable from the Sgr stream stars, but are on highly eccentric orbits
with median $r_{\rm apo} \sim $ 25 kpc. These stars are likely the `accreted'
halo population thought to be the result of a separate merger with the MW 8-11
Gyr ago. We also find one hypervelocity star candidate. We conclude that Sgr
was enriched to [Fe/H] $\sim$ -0.2 before its most recent pericenter passage.
If the `accreted halo' population is from one major accretion event, then this
progenitor galaxy was enriched to at least [Fe/H] $\sim$ -0.6, and had a
similar star formation history to Sgr before merging.
