  We have identified eight to ten new cool white dwarfs from the Large Area
Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT)
Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan
Digital Sky Survey (SDSS) to obtain proper motions and a broad ugrizYJHK
wavelength coverage. Optical spectroscopic observations were secured at Gemini
Observatory and confirm the degenerate status for eight of our targets. The
final sample includes two additional white dwarf candidates with no
spectroscopic observations. We rely on improved 1D model atmospheres and new
multi-dimensional simulations with CO5BOLD to review the stellar parameters of
the published LAS white dwarf sample along with our additional discoveries.
Most of the new objects possess very cool atmospheres with effective
temperatures below 5000 K, including two pure-hydrogen remnants with a cooling
age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km/s <
vtan < 60 km/s. They are likely thick disk 10-11 Gyr-old objects. In addition
we find a resolved double degenerate system with vtan ~ 155 km/s and a cooling
age between 3.0 and 5.0 Gyr. These white dwarfs could be disk remnants with a
very high velocity or former halo G stars. We also compare the LAS sample with
earlier studies of very cool degenerates and observe a similar deficit of
helium-dominated atmospheres in the range 5000 < Teff (K) < 6000. We review the
possible explanations for the spectral evolution from helium-dominated towards
hydrogen-rich atmospheres at low temperatures.
