The Chemical Composition of Extreme-Velocity Stars
  Little is known about the origin of the fastest stars in the Galaxy. Our
understanding of the Milky Way and surrounding dwarf galaxies chemical
evolution history allows us to use the chemical composition of a star to
investigate its origin, and say whether a star was formed in-situ or was
accreted. However, the fastest stars, the hypervelocity stars, are young and
massive and their chemical composition has not yet been analyzed. 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. We have used high-resolution
ARCES/3.5m Apache Point Observatory, MIKE/Magellan spectra to study the
chemical details of 15 late-type hypervelocity stars candidates. 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$^{-1}$. Therefore, our sample stars are not fast enough to be classified as
Hypervelocity stars, and are what is known as extreme-velocity stars. Our
sample has a wide iron abundance range of $-2.5 \le \mathrm{[Fe/H]} \le -0.9$.
Their chemistry indicate that at least 50\% of them are accreted extragalactic
stars, with iron-peak elements consistent with prior sub-Chandrasekhar mass
type Ia supernova enrichment. Without indication of binary companions, their
chemical abundances and orbital parameters are indicative that they are the
accelerated tidal debris of disrupted dwarf galaxies.
