We report the discovery of an unbound hyper-velocity star, US 708, in the
Milky Way halo, with a heliocentric radial velocity of +708+-15km/s. A
quantitative NLTE model atmosphere analysis of optical spectra obtained with
LRIS at the Keck I telescope shows that US 708 is an extremely helium-rich
(N(He)/N(H)=10) subluminous O type star with Teff=44500K, log g=5.23 at a
distance of 19kpc. Its Galactic rest frame velocity is at least 751km/s, much
higher than the local Galactic escape velocity indicating that the star is
unbound to the Galaxy. It has been suggested that such hyper-velocity stars can
be formed by the tidal disruption of a binary through interaction with the
super-massive black hole (SMBH) at the Galactic centre (GC). Numerical
kinematical experiments are carried out to reconstruct the path from the GC.U S
708 needs about 32Myrs to travel from the GC to its present position, less than
its evolutionary lifetime. Its predicted proper motion mue(alpha)
cos(delta)=-2.3mas/y and mue(delta)=-2.4mas/y should be measurable by future
space missions. We conjecture that US 708 is formed by the merger of two helium
white dwarfs in a close binary induced by the interaction with the SMBH in the
GC and then escaped.Comment: 4 pages, 3 figure