Multi-Gigayear White Dwarf Cooling Delays from Clustering-Enhanced
  Gravitational Sedimentation
  Cooling white dwarfs (WDs) can yield accurate ages when theoretical cooling
models fully account for the physics of the dense plasma of WD interiors. We
use MESA to investigate cooling models for a set of massive and ultra-massive
WDs (0.9-1.3 $M_\odot$) for which previous models fail to match kinematic age
indicators based on Gaia DR2. We find that the WDs in this population can be
explained as C/O cores experiencing unexpectedly rapid $^{22}$Ne sedimentation
in the strongly liquid interior just prior to crystallization. We propose that
this rapid sedimentation is due to the formation of solid clusters of $^{22}$Ne
in the liquid C/O background plasma. We show that these heavier solid clusters
sink faster than individual $^{22}$Ne ions and enhance the sedimentation
heating rate enough to dramatically slow WD cooling. MESA models including our
prescription for cluster formation and sedimentation experience cooling delays
of $\approx$4 Gyr on the WD Q branch, alleviating tension between cooling ages
and kinematic ages. This same model then predicts cooling delays coinciding
with crystallization of 6 Gyr or more in lower mass WDs (0.6-0.8 $M_\odot$).
Such delays are compatible with, and perhaps required by, observations of WD
populations in the local 100 pc WD sample and the open cluster NGC 6791. These
results motivate new investigations of the physics of strongly coupled C/O/Ne
plasma mixtures in the strongly liquid state near crystallization and tests
through comparisons with observed WD cooling.
