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
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.
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.
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.
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.
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.
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.
By Kerry Hensley
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
