AAS Nova

Crystallization, Convection, and a Magnetic White Dwarf Mystery

By on Features
Share:
photograph of a white dwarf
A white dwarf shines brightly at the center of this Hubble Space Telescope image. [NASA, ESA, P. McGill (Univ. of California, Santa Cruz and University of Cambridge), K. Sahu (STScI), J. Depasquale (STScI); CC BY 4.0]

Most stars in the Milky Way will evolve into white dwarfs: ultra-hot, crystallized stellar cores, some of which have magnetic fields millions of times stronger than Earth’s. Could the crystallization of white dwarf interiors explain why some of these stars have such strong magnetic fields?

Magnetic Mystery

Hubble image of the Ring Nebula

When a super-hot white dwarf illuminates the diffuse shells of gas that surround it, we see a glowing planetary nebula. The central white dwarf is visible in this image of the Ring Nebula. [NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

Roughly 5–6 billion years from now, the Sun will cease all nuclear fusion in its core and cast off the outer layers of its atmosphere. Left behind will be a blazingly hot, Earth-sized core of carbon and oxygen wreathed in a colorful and ephemeral planetary nebula. This carbon–oxygen core — a white dwarf — will slowly cool over trillions of years and fade from view. Such is the fate of more than 95% of the stars in our galaxy.

Some white dwarfs have extremely strong magnetic fields, and the origin of these fields isn’t yet clear. Though the magnetic fields in question are a million times stronger than Earth’s, they might form in similar ways, as new research from José Rafael Fuentes (University of Colorado Boulder) and collaborators shows.

Creating Crystal Interiors

Many magnetic fields in the universe, including Earth’s, form in liquids that have three properties: they’re electrically conductive, they rotate, and they convect — rising and falling like the globs of wax in a lava lamp. As white dwarfs begin to cool, a process begins by which their liquid interiors may achieve all three criteria necessary to generate a magnetic field.

plot of composition flux as a function of time

The composition flux, τ, as a function of time for a 0.9-solar-mass white dwarf. Convection of the white dwarf’s liquid layer is only efficient while the composition flux is large. [Fuentes et al. 2024]

When first formed, white dwarfs are filled with a hot quantum liquid of carbon and oxygen. As they cool, their centers crystallize into a solid, with a layer of quantum liquid surrounding the crystal core. Crystallization changes the composition of the interior, as oxygen tends to be pulled into the crystal core and carbon tends to remain in the liquid. The difference in chemical makeup causes the electrically conductive, rotating fluid to convect — setting the stage for magnetic-field creation.

To probe whether crystallization could help create the million-Gauss magnetic fields seen in some white dwarfs, Fuentes and collaborators modeled the interiors of white dwarfs as they crystallize. The team used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution model to show that during a brief, 10-million-year period, strong convection could generate magnetic fields of 1–100 million Gauss.

Plot of modeled and observed magnetic fields strengths of white dwarfs

Comparison of the magnetic field strengths obtained though modeling (blue line) with the observed magnetic fields of white dwarfs (symbols). The filled symbols show white dwarfs that are expected to be crystallizing, given their ages, while the open symbols show white dwarfs that are likely not yet crystallizing. Click to enlarge. [Adapted from Fuentes et al. 2024]

Short Phase, Lasting Consequences

While the period of strong convection that creates magnetic fields is short lived, the magnetic field is likely to be long lasting; it takes a long time for magnetic fields to dissipate in a white dwarf, especially once it crystallizes completely.

The models used by Fuentes and coauthors reproduce some observed properties of white-dwarf magnetic fields, such as the lack of a dependence of the field strength on the rotation rate. However, the models also predict that magnetic fields should be stronger for more massive white dwarfs, which observations don’t support. Extending the modeling forward in time may reveal how the magnetic fields evolve and diffuse as the star cools, helping to make sense of these magnetic crystalline stars.

Citation

“A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs,” J. R. Fuentes et al 2024 ApJL 964 L15. doi:10.3847/2041-8213/ad3100