New spectroscopic observations of the halo hyper-velocity star candidate SDSS J121150.27+143716.2 ( V = 17.92 mag ) revealed a cool companion to the hot subdwarf primary .
The components have a very similar radial velocity and their absolute luminosities are consistent with the same distance , confirming the physical nature of the binary , which is the first double-lined hyper-velocity candidate .
Our spectral decomposition of the Keck/ESI spectrum provided an sdB+K3V pair , analogous to many long-period subdwarf binaries observed in the Galactic disk .
We found the subdwarf atmospheric parameters : T _ { eff } = 30 600 \pm 500 K , \log { g } = 5.57 \pm 0.06 cm s ^ { -2 } and He abundance \log ( n { He } / n { H } ) = -3.0 \pm 0.2 .
Oxygen is the most abundant metal in the hot subdwarf atmosphere , and Mg and Na lines are the most prominent spectral features of the cool companion , consistent with a metallicity of [ { Fe } / { H } ] = -1.3 .
The non-detection of radial velocity variations suggest the orbital period to be a few hundred days , in agreement with similar binaries observed in the disk .
Using the SDSS-III flux calibrated spectrum we measured the distance to the system d = 5.5 \pm 0.5 kpc , which is consistent with ultraviolet , optical , and infrared photometric constraints derived from binary spectral energy distributions .
Our kinematic study shows that the Galactic rest-frame velocity of the system is so high that an unbound orbit can not be ruled out .
On the other hand , a bound orbit requires a massive dark matter halo .
We conclude that the binary either formed in the halo or it was accreted from the tidal debris of a dwarf galaxy by the Milky Way .