Using chemical tagging to redefine the interface of the Galactic disc 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 Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. The main aim of our analysis is to explore the Galactic disc-halo transition region within -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, 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 disc and halo are not chemically distinct in all elements indicating a smooth transition between the thick disc and halo; (3) a subsample of the alpha-poor stars at metallicities as low as [Fe/H] similar to-0.85 dex are chemically and dynamically consistent with the thin disc indicating that the thin disc may extend to lower metallicities than previously thought; and (4) the locations of the most metal-poor thin disc 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.