Disentangling the Galactic Halo with APOGEE: II. Chemical and Star
  Formation Histories for the Two Distinct Populations
  The formation processes that led to the current Galactic stellar halo are
still under debate. Previous studies have provided evidence for different
stellar populations in terms of elemental abundances and kinematics, pointing
to different chemical and star-formation histories. In the present work we
explore, over a broader range in metallicity (-2.2 < [Fe/H] < -0.5), the two
stellar populations detected in the first paper of this series from metal-poor
stars in DR13 of the Apache Point Observatory Galactic Evolution Experiment
(APOGEE). We aim to infer signatures of the initial mass function (IMF) and the
most APOGEE-reliable alpha-elements (O, Mg, Si and Ca). Using simple
chemical-evolution models, for each population. Compared with the low-alpha
population, we obtain a more intense and longer-lived SFH, and a top-heavier
IMF for the high-alpha population.
