Discovery of a Nearby Halo White Dwarf with Proper Motion (mu) = 2n55 yr^-1

We report the discovery of PM J13420-3415, a faint (V = 17) white dwarf with a very high proper motion (mu)= 2n55 yr^-1. The star was found in the southern sky extension of the SUPERBLINK propermotion survey. A red spectrum shows the classical signature of a DA white dwarf, with a weak H(alpha) line in absorption as the only prominent feature. The star is also found to have a large radial velocity Vrad = +212 +- 15 km s^-1. At the adopted distance of 18 pc, the star has a very large space motion of 313 km s-1 relative to the Sun. An integration of the space motion shows that the star is on a nearly polar Galactic orbit and is thus an unambiguous member of the Galactic halo. However, with an estimated effective temperature 5000 K < Teff < 5500 K, the white dwarf appears to be much younger than expected for a denizen of the halo. The apparent paradox can be explained if the white dwarf is the relatively young (~2 Gyr) remnant of a longer lived (10-14 Gyr) main-sequence star, in which case the object is predicted to be a low-mass white dwarf with M ~~0.45 M(circle dot operator).