We find two chemically distinct populations separated relatively cleanly in the [ Fe/H ] - [ Mg/Fe ] plane , but also distinguished in other chemical planes , among metal-poor stars ( primarily with metallicities [ Fe/H ] < -0.9 ) observed by the Apache Point Observatory Galactic Evolution Experiment ( APOGEE ) and analyzed for Data Release 13 ( DR13 ) of the Sloan Digital Sky Survey .
These two stellar populations show the most significant differences in their [ X/Fe ] ratios for the \alpha -elements , C+N , Al , and Ni .
In addition to these populations having differing chemistry , the low metallicity high-Mg population ( which we denote the HMg population ) exhibits a significant net Galactic rotation , whereas the low-Mg population ( or LMg population ) has halo-like kinematics with little to no net rotation .
Based on its properties , the origin of the LMg population is likely as an accreted population of stars .
The HMg population shows chemistry ( and to an extent kinematics ) similar to the thick disk , and is likely associated with in situ formation .
The distinction between the LMg and HMg populations mimics the differences between the populations of low- and high- \alpha halo stars found in previous studies , suggesting that these are samples of the same two populations .