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.