The Fastest Travel Together: Chemical Tagging of the Fastest Stars in
  Gaia DR2 to the Stellar Halo
  The fastest moving stars provide insight into several fundamental properties
of the Galaxy, including the escape velocity as a function of Galactocentric
radius, the total mass, and the nature and frequency of stellar encounters with
the central supermassive black hole. The recent second data release of Gaia has
allowed the identification of new samples of stars with extreme velocities.
Discrimination among the possible origins of these stars is facilitated by
chemical abundance information. We here report the results from our
high-resolution spectroscopic followup, using the Apache Point Observatory, of
five late-type `hypervelocity' star candidates, characterised by total Galactic
rest-frame velocities between 500-600 km/s and estimated to have a probability
larger than 50% to be unbound from the Milky Way. Our new results confirm the
Gaia DR2 radial velocities to within 1 km/s. We derived stellar atmospheric
parameters and chemical abundances for several species including
$\alpha$-elements (Mg, Ti, Si, Ca), Fe-peak elements (Fe, Ni, Co, Cr, Mn),
neutron-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu) and odd-Z elements (Na,
Al, K, V, Cu, Sc). We find that all stars observed are metal-poor giants with
-2 $\leq$ [Fe/H] $\leq$ -1 dex and are chemically indistinguishable from
typical halo stars. Our results are supported by the chemical properties of
four additional stars with extreme space motions which were observed by
existing spectroscopic surveys. We conclude that these stars are simply the
high-velocity tail of the stellar halo and effectively rule out more exotic
origins such as from the Galactic centre or the Large Magellanic Cloud.
