Using chemical tagging to redefine the interface of the Galactic disk
  and halo
  We present a chemical abundance distribution study in 14 $\alpha$, odd-Z,
even-Z, light, and Fe-peak elements of approximately 3200 intermediate
metallicity giant stars from the APOGEE survey. The main aim of our analysis is
to explore the Galactic disk-halo transition region between -1.20 $<$ [Fe/H]
$<$ -0.55 as a means to study chemical difference (and similarities) between
these components. In this paper, we show that there is an $\alpha$-poor and
$\alpha$-rich sequence within both the metal-poor and intermediate metallicity
regions. Using the Galactic rest-frame radial velocity and spatial positions,
we further separate our sample into the canonical Galactic components. We then
studied the abundances ratios, of Mg, Ti, Si, Ca, O, S, Al, C+N, Na, Ni, Mn, V,
and K for each of the components and found the following: (1) the $\alpha$-poor
halo subgroup is chemically distinct in the $\alpha$-elements (particularly O,
Mg, and S), Al, C+N, and Ni from the $\alpha$-rich halo, consistent with the
literature confirming the existence of an $\alpha$-poor accreted halo
population; (2) the canonical thick disk and halo are not chemically distinct
in all elements indicating a smooth transition between the thick disk and halo;
(3) a subsample of the $\alpha$-poor stars at metallicities as low as [Fe/H]
$\sim$ -0.85 dex are chemically and dynamically consistent with the thin disk
indicating that the thin disk may extend to lower metallicities than previously
thought, and (4) that the location of the most metal-poor thin disk stars are
consistent with a negative radial metallicity gradient. Finally, we used our
analysis to suggest a new set of chemical abundance planes ([$\alpha$/Fe],
[C+N/Fe], [Al/Fe], and [Mg/Mn]) that may be able to chemically label the
Galactic components in a clean and efficient way independent of kinematics.
