Context :

Aims : We selected the bluest object , WISE J0725 - 2351 , from Luhman ’ s new high proper motion ( HPM ) survey based on observations with the Wide-field Infrared Survey Explorer ( WISE ) for spectroscopic follow-up observations .
Our aim was to unravel the nature of this relatively bright ( V \sim 12 , J \sim 11 ) HPM star ( \mu = 267 mas/yr ) .

Methods : We obtained low- and medium-resolution spectra with the European Southern Observatory ( ESO ) New Technology Telescope ( NTT ) /EFOSC2 and Very Large Telescope ( VLT ) /XSHOOTER instruments , investigated the radial velocity and performed a quantitative spectral analysis that allowed us to determine physical parameters .
The fit of the spectral energy distribution based on the available photometry to low-metallicity model spectra and the similarity of our target to a metal-poor benchmark star ( HD 84937 ) allowed us to estimate the distance and space velocity .

Results : As in the case of HD 84937 , we classified WISE J0725 - 2351 as sdF5 : or a metal-poor turnoff star with [ Fe / H ] = - 2.0 \pm 0.2 , T _ { eff } = 6250 \pm 100 K , \log { g } = 4.0 \pm 0.2 , and a possible age of about 12 Gyr .
At an estimated distance of more than 400 pc , its proper motion translates to a tangential velocity of more than 500 km/s .
Together with its constant ( on timescales of hours , days , and months ) and large radial velocity ( about + 240 km/s ) , the resulting Galactic restframe velocity is about 460 km/s , implying a bound retrograde orbit for this extreme halo object that currently crosses the Galactic plane at high speed .

Conclusions :