Neon Cluster Formation and Phase Separation During White Dwarf Cooling

TL;DR

This study uses molecular dynamics simulations to investigate whether $^{22}$Ne can form clusters in white dwarf interiors, concluding it cannot, thus challenging previous explanations for anomalous cooling observed in some white dwarfs.

Contribution

The paper demonstrates through simulations that $^{22}$Ne cannot cluster in C/O white dwarfs, providing a phase diagram and ruling out cluster sedimentation as a heat source.

Findings

$^{22}$Ne does not form clusters in C/O white dwarfs.

Strong $^{22}$Ne enrichment is not possible below 30% abundance.

Anomalous cooling likely due to unusual compositions, not $^{22}$Ne sedimentation.

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.