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(22)Ne sedimentation has long been considered a possible heat source, recent work suggests that(22)Ne must separate into clusters, enhancing diffusion, in order for sedimentation to provide heating on the observed timescale. We show definitively that(22)Ne 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(22)Ne enrichment is not achievable for(22)Ne abundance less than or similar to 30%. We conclude that the anomalous heating cannot be due to(22)Ne cluster sedimentation and that Q branch WDs may have an unusual composition, possibly rich with heavier elements.