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 .