Forging Neon-distilling White Dwarfs in the Stellar Engulfments of Helium White Dwarfs

Abstract Once carbon–oxygen white dwarfs cool sufficiently, they crystallize from the inside out. If the white dwarf is rich enough in 22 Ne, 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 22 Ne 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 22 Ne 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 22 Ne abundances high enough to distill 22 Ne. This enhancement occurs because carbon dredged up following an especially energetic and off-center helium flash can be processed into 22 Ne by subsequent hydrogen shell burning and helium shell burning. 22 Ne-distilling white dwarfs from these merger channels are predicted to be somewhat more massive than typical white dwarfs (up to ≃0.7 M ⊙ ) and may have anomalous rotation rates, consistent with DAHe white dwarfs. These binary formation channels for 22 Ne-rich white dwarfs reveal new connections between binary interactions and white dwarf cooling phenomena.