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 ultramassive WDs (0.9-1.3 M-circle dot) for which previous models have failed 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 Ne-22 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 Ne-22 in the liquid C/O background plasma. We show that these heavier solid clusters sink faster than individual Ne-22 ions and enhance the sedimentation heating rate enough to dramatically slow WD cooling. MESAmodels including our prescription for cluster formation and sedimentation experience cooling delays of 4 Gyr on the WDQ 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-circle dot). 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.