Hills coined this term to describe stars ejected with total velocities exceeding 1000 km s −1 after a stellar encounter with a massive black hole. The existence of hypervelocity stars (HVSs) was first proposed by Hills ( 1988). Without indication of binary companions, their chemical abundances and orbital parameters indicate that they are the accelerated tidal debris of disrupted dwarf galaxies. Their chemistry indicates that at least 50% of them are accreted extragalactic stars, with iron-peak elements consistent with prior enrichment by sub-Chandrasekhar mass Type Ia supernovae. Our sample has a wide iron abundance range of −2.5 ≤ ≤ −0.9. Therefore, our sample stars are not fast enough to be classified as hypervelocity stars, and are what is known as extreme-velocity stars. With Gaia EDR3 astrometry and spectroscopically determined radial velocities we found total velocities with a range of 274–520 km s −1 and mean value of 381 km s −1. We have used high-resolution ARCES/3.5 m Apache Point Observatory, MIKE/Magellan spectra to study the chemical details of 15 late-type hypervelocity star candidates. Though it is difficult to analyze the chemical composition of a massive young star, we are well versed in the analysis of late-type stars. However, the fastest stars, the hypervelocity stars, are young and massive and their chemical composition has not yet been analyzed. Our understanding of the chemical evolution history of the Milky Way and surrounding dwarf galaxies allows us to use the chemical composition of a star to investigate its origin and to say whether it was formed in situ or was accreted. Little is known about the origin of the fastest stars in the Galaxy.
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