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

TL;DR

This study uses high-resolution spectroscopy to analyze the chemical compositions of the fastest stars in Gaia DR2, confirming they are typical halo stars and not from exotic origins like the Galactic center.

Contribution

It provides the first detailed chemical abundance analysis of hypervelocity star candidates, confirming their halo origin and ruling out more exotic formation scenarios.

Findings

Stars are metal-poor giants with halo-like chemical signatures.

Gaia DR2 velocities are confirmed with high precision.

Stars are consistent with the high-velocity tail of the stellar halo.

Abstract

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.