Buoyant crystals halt the cooling of white dwarf stars

TL;DR

This paper proposes that buoyant crystals formed during white dwarf cooling create a distillation process that releases gravitational energy, explaining the observed delayed luminosity and challenging the view of white dwarfs as dead stars.

Contribution

It introduces a new mechanism involving buoyant crystal formation and distillation that halts white dwarf cooling for billions of years, aligning theory with observations.

Findings

Distillation causes prolonged white dwarf luminosity plateau.

Buoyant crystals explain the delayed cooling population.

White dwarfs can have luminosities exceeding some main-sequence stars.

Abstract

White dwarfs are stellar remnants devoid of a nuclear energy source, gradually cooling over billions of years and eventually freezing into a solid state from the inside out. Recently, it was discovered that a population of freezing white dwarfs maintains a constant luminosity for a duration comparable to the age of the universe, signaling the presence of a powerful yet unknown energy source that inhibits the cooling. For certain core compositions, the freezing process is predicted to trigger a solid-liquid distillation mechanism, due to the solid phase being depleted in heavy impurities. The crystals thus formed are buoyant and float up, thereby displacing heavier liquid downward and releasing gravitational energy. Here we show that distillation interrupts the cooling for billions of years and explains all the observational properties of the unusual delayed population. With a steady luminosity surpassing that of some main-sequence stars, these white dwarfs defy their conventional portrayal as dead stars. Our results highlight the existence of peculiar merger remnants and have profound implications for the use of white dwarfs in dating stellar populations.