| Summary: | Gaia's exquisite parallax measurements allowed for the discovery and characterization of the Q branch in the Hertzsprung–Russell diagram, where massive C/O white dwarfs (WDs) pause their dimming due to energy released during crystallization. Interestingly, the fraction of old stars on the Q branch is significantly higher than in the population of WDs that will become Q branch 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 longer cooling delay than that of standard crystallizing WDs. Previous attempts to explain this cooling anomaly have invoked mechanisms involving supersolar initial metallicities. In this paper, we describe a novel scenario in which a standard composition WD merges with a subgiant star. The evolution of the resulting merger remnant leads to the creation of a large amount of ^26 Mg, which, along with the existing ^22 Ne, undergoes a distillation process that can release enough energy to explain the Q branch cooling problem without the need for atypical initial abundances. The anomalously high number of old stars on the Q branch may thus be evidence that mass transfer from subgiants to WDs leads to unstable mergers.
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