Explaining the chemical trajectories of accreted and in-situ halo stars
  of the Milky Way
  The Milky Way underwent its last significant merger ten billion years ago,
when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and
progenitor stars born prior to the merger make up the bulk of the inner halo.
Even though these two main populations of halo stars have similar $durations$
of star formation prior to their merger, they differ in [$\alpha$/Fe]-[Fe/H]
space, with the GES population bending to lower [$\alpha$/Fe] at a relatively
low value of [Fe/H]. We use cosmological simulations of a 'Milky Way' to argue
that the different tracks of the halo stars through the [$\alpha$/Fe]-[Fe/H]
plane are due to a difference in their star formation history and efficiency,
with the lower mass GES having its low and constant star formation regulated by
feedback whilst the higher mass main progenitor has a higher star formation
rate prior to the merger. The lower star formation efficiency of GES leads to
lower gas pollution levels, pushing [$\alpha$/Fe]-[Fe/H] tracks to the left. In
addition, the increasing star formation rate maintains a higher relative
contribution of Type~II SNe to Type~Ia SNe for the main progenitor population
that formed during the same time period, thus maintaining a relatively high
[$\alpha$/Fe]. Thus the different positions of the downturns in the
[$\alpha$/Fe]-[Fe/H] plane for the GES stars are not reflective of different
star formation durations, but instead reflect different star formation
efficiencies. We argue that cosmological simulations match a wide range of
independent observations, breaking degeneracies that exist in simpler models.
