Neon Cluster Formation and Phase Separation during White Dwarf Cooling

Recent observations of Galactic white dwarfs (WDs) with Gaia suggest there is a population of massive crystallizing WDs exhibiting anomalous cooling—the Q branch. While single-particle 22Ne sedimentation has long been considered a possible heat source, recent work suggests that 22Ne must separate into clusters, enhancing diffusion, in order for sedimentation to provide heating on the observed timescale. We show definitively that 22Ne cannot separate to form clusters in C/O WDs using molecular dynamics simulations, and we further present a general C/O/Ne phase diagram showing that strong 22Ne enrichment is not achievable for 22Ne abundance ≲30%. We conclude that the anomalous heating cannot be due to 22Ne cluster sedimentation and that Q branch WDs may have an unusual composition, possibly rich with heavier elements.