Neon Cluster Formation and Phase Separation During White Dwarf Cooling

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 $\lesssim 30\%$. 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.

7 pages, 4 figures, accepted for publication in ApJ Letters