Forever young white dwarfs: when stellar ageing stops

TL;DR

This paper proposes that $^{22}$Ne sedimentation in ultramassive white dwarfs with CO cores explains their unexpectedly long cooling delays observed by Gaia, suggesting the existence of CO-core ultramassive white dwarfs.

Contribution

It introduces a new explanation for prolonged white dwarf cooling delays via $^{22}$Ne sedimentation, supporting the existence of CO-core ultramassive white dwarfs contrary to standard models.

Findings

$^{22}$Ne sedimentation accounts for observed cooling delays.

Population synthesis aligns Gaia data with CO-core ultramassive white dwarfs.

High $^{22}$Ne abundance challenges standard stellar evolution models.

Abstract

White dwarf stars are the most common end point of stellar evolution. Of special interest are the ultramassive white dwarfs, as they are related to type Ia Supernovae explosions, merger events, and Fast Radio Bursts. Ultramassive white dwarfs are expected to harbour oxygen-neon (ONe) cores as a result of single standard stellar evolution. However, a fraction of them could have carbon-oxygen (CO) cores. Recent studies, based on the new observations provided by the {\it Gaia} space mission, indicate that a small fraction of the ultramassive white dwarfs experience a strong delay in their cooling, which cannot be attributed only to the occurrence of crystallization, thus requiring an unknown energy source able to prolong their life for long periods of time. In this study we find that the energy released by $^{22}$Ne sedimentation in the deep interior of ultramassive white dwarfs with CO cores and high $^{22}$Ne content is consistent with the long cooling delay of these stellar remnants. On the basis of a synthesis study of the white dwarf population, based on Monte Carlo techniques, we find that the observations revealed by {\it Gaia} can be explained by the existence of these prolonged youth ultramassive white dwarfs. Although such a high $^{22}$Ne abundance is not consistent with the standard evolutionary channels, our results provide sustain to the existence of CO-core ultramassive white dwarfs and to the occurrence of $^{22}$Ne sedimentation.