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Gaia's exquisite parallax measurements allowed for the discovery andcharacterization of the Q branch in the Hertzsprung-Russell diagram, wheremassive C/O white dwarfs (WDs) pause their dimming due to energy releasedduring crystallization. Interestingly, the fraction of old stars on the Qbranch is significantly higher than in the population of WDs that will become Qbranch stars or that were Q branch stars in the past. From this, Cheng et al.inferred that ~6% of WDs passing through the Q branch experience a much longercooling delay than that of standard crystallizing WDs. Previous attempts toexplain this cooling anomaly have invoked mechanisms involving super-solarinitial metallicities. In this paper, we describe a novel scenario in which astandard composition WD merges with a subgiant star. The evolution of theresulting merger remnant leads to the creation of a large amount of 26Mg,which, along with the existing 22Ne, undergoes a distillation process that canrelease enough energy to explain the Q branch cooling problem without the needfor atypical initial abundances. The anomalously high number of old stars onthe Q branch may thus be evidence that mass transfer from subgiants to WDsleads to unstable mergers.

The Q Branch Cooling Anomaly Can Be Explained by Mergers of White Dwarfs and Subgiant Stars