A cooling anomaly of high-mass white dwarfs
  Recently, the power of Gaia data has revealed an enhancement of high-mass
white dwarfs (WDs) on the Hertzsprung--Russell diagram, called the Q branch.
This branch is located at the high-mass end of the recently identified
crystallization branch. Investigating its properties, we find that the number
density and velocity distribution on the Q branch cannot be explained by the
cooling delay of crystallization alone, suggesting the existence of an extra
cooling delay. To quantify this delay, we statistically compare two age
indicators -- the dynamical age inferred from transverse velocity, and the
photometric isochrone age -- for more than one thousand high-mass WDs
(1.08--1.23 $M_\odot$) selected from Gaia Data Release 2. We show that about 6
% of the high-mass WDs must experience an 8 Gyr extra cooling delay on the Q
branch, in addition to the crystallization and merger delays. This cooling
anomaly is a challenge for WD cooling models. We point out that $^{22}$Ne
settling in C/O-core WDs could account for this extra cooling delay.
