A star whose velocity is so great that it will escape the
→ gravitational potential of our
→ Galaxy. Depending on the location and direction of
motion, this criterion typically corresponds to a stellar velocity in
the Galactic → rest frame larger than
400 km s-1, and up to about 1200 km s-1.
The nature of the HVSs spans a wide range of types from
→ OB stars, to metal-poor
→ F-type stars and G/K dwarfs. While there is evidence from many
late-type B HVSs in the → halo
to originate from the Galactic
→ supermassive black hole (SMBH),
other HVSs seem to originate from the → galactic disk.
HVSs can obtain their large velocities from a number of different processes:
→ Tidal disruption of
→ close binary stars by the central SMBH of the Milky Way.
In this process one star is captured by the SMBH while the other is ejected at
high speed via the → gravitational slingshot mechanism.
Exchange encounters in other dense stellar environments
between hard binaries (→ hard binary)
and → massive stars may cause
stars to be ejected and escape our Galaxy.
Disruption of close binaries via → supernova explosions.
The → runaway velocities of both
ejected stars can reach large values when asymmetric supernovae are
considered, i.e. when the newborn → neutron star
receives a momentum kick at birth.
(see, e.g., T. M. Tauris, 2014, and references therein, arXiv:1412.0657).
See also: → hyper-; → velocity;
→ star.
