  A recent analysis of the 100 pc white dwarf sample in the SDSS footprint
demonstrated for the first time the existence of a well defined ultracool -- or
IR-faint -- white dwarf sequence in the Hertzsprung-Russell diagram. Here we
take advantage of this discovery to enlarge the IR-faint white dwarf sample
threefold. We expand our selection to the entire Pan-STARRS survey footprint as
well as the Montreal White Dwarf Database 100 pc sample, and identify 37
candidates with strong flux deficits in the optical. We present follow-up
Gemini optical spectroscopy of 30 of these systems, and confirm all of them as
IR-faint white dwarfs. We identify an additional set of 33 objects as
candidates based on their colors and magnitudes. We present a detailed model
atmosphere analysis of all 70 newly identified IR-faint white dwarfs together
with 35 previously known objects reported in the literature. We discuss the
physics of model atmospheres and show that the key physical ingredient missing
in our previous generation of model atmospheres was the high-density correction
to the He-minus free-free absorption coefficient. With new model atmospheres
calculated for the purpose of this analysis, we now obtain significantly higher
effective temperatures and larger stellar masses for these IR-faint white
dwarfs than the Teff and M values reported in previous analyses, thus solving a
two decade old problem. In particular, we identify in our sample a group of
ultramassive white dwarfs in the Debye cooling phase with stellar parameters
never measured before.

Solar and Stellar Astrophysics