Discovery of a nearby halo white dwarf with proper motion μ=2".55 yr-1

We report the discovery of PM J13420 - 3415, a faint (V = 17) white dwarf with a very high proper motion mu = 2.'' 55 yr(-1). The star was found in the southern sky extension of the SUPERBLINK proper-motion 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 V-rad = +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, the white dwarf 5000 K < T-eff < 5500 K 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 (similar to 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 approximate to 0.45 M-..