The 100 pc White Dwarf Sample in the SDSS Footprint
  We present follow-up spectroscopy of 711 white dwarfs within 100 pc, and
present a detailed model atmosphere analysis of the 100 pc white dwarf sample
in the SDSS footprint. Our spectroscopic follow-up is complete for 83% of the
white dwarfs hotter than 6000 K, where the atmospheric composition can be
constrained reliably. We identify 1508 DA white dwarfs with pure hydrogen
atmospheres. The DA mass distribution has an extremely narrow peak at
$0.59~M_{\odot}$, and reveals a shoulder from relatively massive white dwarfs
with $M=0.7$-$0.9~M_{\odot}$. Comparing this distribution with binary
population synthesis models, we find that the contribution from single stars
that form through mergers cannot explain the over-abundance of massive white
dwarfs. In addition, the mass distribution of cool DAs shows a near absence of
$M>1~M_{\odot}$ white dwarfs. The pile-up of 0.7-$0.9~M_{\odot}$ and the
disappearance of $M>1~M_{\odot}$ white dwarfs is consistent with the effects of
core crystallization. Even though the evolutionary models predict the location
of the pile-up correctly, the delay from the latent heat of crystallization by
itself is insufficient to create a significant pile-up, and additional cooling
delays from related effects like phase separation are necessary. We also
discuss the population of infrared-faint (ultracool) white dwarfs, and
demonstrate for the first time the existence of a well defined sequence in
color and magnitude. Curiously, this sequence is connected to a region in the
color-magnitude diagrams where the number of helium-dominated atmosphere white
dwarfs is low. This suggests that the infrared-faint white dwarfs likely have
mixed H/He atmospheres.
