Visitors from the Halo: 11 Gyr old White Dwarfs in the Solar Neighborhood

We report the discovery of three nearby old halo white dwarf candidates in the Sloan Digital Sky Survey (SDSS), including two stars in a common proper motion binary system. These candidates are selected from our 2800 square degree proper motion survey on the Bok and U.S. Naval Observatory Flagstaff Station 1.3m telescopes, and they display proper motions of 0.4-0.5 arcsec/yr. Follow-up MMT spectroscopy and near-infrared photometry demonstrate that all three objects are hydrogen-dominated atmosphere white dwarfs with Teff = 3700 - 4100 K. For average mass white dwarfs, these temperature estimates correspond to cooling ages of 9-10 Gyr, distances of 70-80 pc, and tangential velocities of 140-200 km/s. Based on the UVW space velocities, we conclude that they most likely belong to the halo. Furthermore, the combined main-sequence and white dwarf cooling ages are 10-11 Gyr. Along with SDSS J1102+4113, they are the oldest field white dwarfs currently known. These three stars represent only a small fraction of the halo white dwarf candidates in our proper motion survey, and they demonstrate that deep imaging surveys like the Pan-STARRS and Large Synoptic Survey Telescope should find many old thick disk and halo white dwarfs that can be used to constrain the age of the Galactic thick disk and halo.