The delayed evolution of high-mass white dwarfs: the Q branch and double-white-dwarf mergers

Studying high-mass white dwarfs (WDs) can shed light on the progenitors of Type Ia supernovae. Recently, the unprecedented power of Gaia Data Release 2 (DR2) has revealed an enhancement of high-mass WDs on the H-R diagram, called the Q branch. This branch is located at the high-mass end of the crystallization branch identified by Tremblay et al. (2019). However, investigating its properties, we find that the number density and the velocity distribution of WDs on the Q branch cannot be explained by the cooling delay of crystallization alone, suggesting the existence of an extra cooling delay. To explore the properties of this delay, we statistically compare two age indicators -- the dynamical age reflected by transverse velocity and the photometric age -- for more than one thousand high-mass WDs (1.08-1.23 $\rm M_\odot$). We show that, in addition to crystallization and merger delays, an $8$-Gyr cooling delay is required on the Q branch, which affects about $7\%$ of high-mass WDs. This is a challenge to WD cooling models. $\rm ^{22}Ne$ settling in some WDs might account for this extra delay. We also show that $20\pm6\%$ of high-mass WDs originate from double-WD mergers, corresponding to a merger rate of $\rm (2.1\pm0.6)\times10^{-14}\,M_\odot^{-1}\,yr^{-1}$ in their mass range. This is a direct observational constraint on the rate of double-WD mergers, which is a promising channel of Type Ia supernova explosion.