The assembly history of the Galactic inner halo inferred from
  alpha-patterns
  We explore the origin of the observed decline in [O/Fe] (and [Mg/Fe]) with
Galactocentric distance for high-metallicity stars ([Fe/H] > -1.1), based on a
sample of halo stars selected within the Apache Point Observatory Galactic
Evolution Experiment (APOGEE) fourteenth data release (DR14). We also analyse
the characteristics of the [$\alpha$/Fe] distributions in the inner-halo
regions inferred from two zoom-in Milky Way mass-sized galaxies that are taken
as case studies. One of them qualitatively reproduces the observed trend to
have higher fraction of $\alpha$-rich star for decreasing galactocentric
distance; the other exhibits the opposite trend. We find that stars with [Fe/H]
> -1.1 located in the range [15 - 30] kpc are consistent with formation in two
starbursts, with maxima separated by about ~ 1 Gyr. We explore the
contributions of stellar populations with different origin to the [$\alpha$/Fe]
gradients detected in stars with [Fe/H] > -1.1. Our analysis reveals that the
simulated halo that best matches the observed chemical trends is characterised
by an accretion history involving low to intermediate-mass satellite galaxies
with a short and intense burst of star formation, and contributions from a more
massive satellite with dynamical masses about ~ 10$^{10}$M$_{\odot}$,
distributing low-[$\alpha$/Fe] stars at intermediate radius.
