Hypervelocity stars


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definition 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 hypervelocity stars (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: 1) 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. 2) Exchange encounters in other dense stellar environments between hard binaries and massive stars may cause stars to be ejected and escape our Galaxy. 3) 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.
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