Neon Cluster Formation and Phase Separation during White Dwarf Cooling
M. E. Caplan, C. J. Horowitz, A. Cumming
Abstract
Abstract 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 ≲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.