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The fastest moving stars provide insight into several fundamental propertiesof the Galaxy, including the escape velocity as a function of Galactocentricradius, the total mass, and the nature and frequency of stellar encounters withthe central supermassive black hole. The recent second data release of Gaia hasallowed the identification of new samples of stars with extreme velocities.Discrimination among the possible origins of these stars is facilitated bychemical abundance information. We here report the results from ourhigh-resolution spectroscopic followup, using the Apache Point Observatory, offive late-type `hypervelocity' star candidates, characterised by total Galacticrest-frame velocities between 500-600 km/s and estimated to have a probabilitylarger than 50% to be unbound from the Milky Way. Our new results confirm theGaia DR2 radial velocities to within 1 km/s. We derived stellar atmosphericparameters 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 fromtypical halo stars. Our results are supported by the chemical properties offour additional stars with extreme space motions which were observed byexisting spectroscopic surveys. We conclude that these stars are simply thehigh-velocity tail of the stellar halo and effectively rule out more exoticorigins such as from the Galactic centre or the Large Magellanic Cloud.

The Fastest Travel Together: Chemical Tagging of the Fastest Stars in Gaia DR2 to the Stellar Halo