Ages and kinematics of chemically selected, accreted Milky Way halo
  stars
  We exploit the [Mg/Mn]-[Al/Fe] chemical abundance plane to help identify
nearby halo stars in the 14th data release from the APOGEE survey that have
been accreted on to the Milky Way. Applying a Gaussian Mixture Model, we find a
`blob' of 856 likely accreted stars, with a low disc contamination rate of ~7%.
Cross-matching the sample with the second data release from Gaia gives us
access to parallaxes and apparent magnitudes, which place constraints on
distances and intrinsic luminosities. Using a Bayesian isochrone pipeline, this
enables us to estimate new ages for the accreted stars, with typical
uncertainties of ~20%. Our new catalogue is further supplemented with estimates
of orbital parameters.
  The blob stars span a metallicities between -0.5 to -2.5, and [Mg/Fe] between
-0.1 to 0.5. They constitute ~30% of the metal-poor ([Fe/H] < -0.8) halo at
metallicities of ~-1.4. Our new ages are mainly range between 8 to 13 Gyr, with
the oldest stars the metal-poorest, and with the highest [Mg/Fe] abundance. If
the blob stars are assumed to belong to a single progenitor, the ages imply
that the system merged with our Milky Way around 8 Gyr ago and that star
formation proceeded for ~5 Gyr. Dynamical arguments suggest that such a single
progenitor would have a total mass of ~1011Msun, similar to that found by other
authors using chemical evolution models and simulations. Comparing the scatter
in the [Mg/Fe]-[Fe/H] plane of the blob stars to that measured for stars
belonging to the Large Magellanic Cloud suggests that the blob does indeed
contain stars from only one progenitor.
