2015A&A...574A..96S

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∼12, J∼11) HPM star (µ=267mas/yr). We obtained low- and medium-resolution spectra with the European Southern Observatory (ESO) New Technology Telescope (NTT)/EFOSC2 and Very Large Telescope (VLT)/X-Shooter 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. 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±0.2, T_eff=6250±100K, logg=4.0±0.2, and a possible age of about 12Gyr. At an estimated distance of more than 400pc, its proper motion translates to a tangential velocity of more than 500km/s. Together with its constant (on timescales of hours, days, and months) and large radial velocity (about +240km/s), the resulting Galactic restframe velocity is about 460km/s, implying a bound retrograde orbit for this extreme halo object that currently crosses the Galactic plane at high speed.