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.
