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 from [ ] , characterised by derived total Galactic rest-frame velocities between 500–600 km s ^ { -1 } and estimated , by those authors , 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 ^ { -1 } .
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 .