2026ApJ...998...64R

Once carbon-oxygen white dwarfs cool sufficiently, they crystallize from the inside out. If the white dwarf is rich enough in ²²Ne, these crystallized solids are buoyant and rapidly rise, efficiently liberating potential energy, which may halt the cooling of the white dwarf or power magnetic phenomena. Although this ²²Ne distillation process may explain the cooling anomaly in Q-branch white dwarfs and anomalous emission lines in DAHe white dwarfs, its operation demands unusually high ²²Ne abundances not generically predicted by isolated stellar evolution. We show that the engulfments of helium white dwarfs by both main-sequence and red giant stars can result in carbon-oxygen white dwarfs with ²²Ne abundances high enough to distill ²²Ne. This enhancement occurs because carbon dredged up following an especially energetic and off-center helium flash can be processed into ²²Ne by subsequent hydrogen shell burning and helium shell burning. ²²Ne-distilling white dwarfs from these merger channels are predicted to be somewhat more massive than typical white dwarfs (up to ≃0.7M_⊙) and may have anomalous rotation rates, consistent with DAHe white dwarfs. These binary formation channels for ²²Ne-rich white dwarfs reveal new connections between binary interactions and white dwarf cooling phenomena.