We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey ( UKIDSS ) Large Area Survey ( LAS ) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population .
LSR J0745+2627 has a high proper motion ( 890 mas/yr ) and a high reduced proper motion value in the J band ( H _ { J } = 21.87 ) .
We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination .
LSR J0745+2627 is also detected in the Sloan Digital Sky Survey ( SDSS ) and the Wide-field Infrared Survey Explorer ( WISE ) .
We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope .
A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880 \pm 90 K. This object is the brightest pure-H ultracool white dwarf ( T _ { eff } < 4000 K ) ever identified .
We have constrained the distance ( 24–45 pc ) , space velocities and age considering different surface gravities .
The results obtained suggest that LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the closest ultracool white dwarfs .