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) 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 Ne-22 settling in C/O-core WDs could account for this extra cooling delay.