Researchers have discovered a mechanism that generates huge magnetic fields in white dwarfs. According to an international team of scientists, the dynamo mechanism can explain the incredibly strong magnetic fields in stars – white dwarfs.
One of the most striking phenomena in astrophysics is the presence of magnetic fields. Like Earth, stars and stellar remnants like white dwarfs have only one magnetic field. It is known that the magnetic fields of white dwarfs can be a million times stronger than those of the Earth. However, their origins have remained a mystery since the discovery of the first magnetic white dwarf in the 1970s. Several theories have been proposed, but none of them has been able to explain the different frequency of occurrence of magnetic white dwarfs both in individual stars and in different environments of binary stars.
This uncertainty can be resolved through research by an international group of astrophysicists. So, it was found that a dynamo mechanism, similar to the one that generates magnetic fields on Earth and other planets, can work in white dwarfs and create much stronger fields.
“We have known for a long time that something is missing in our understanding of the magnetic fields of white dwarfs, since the statistics obtained from observations simply did not make sense. The idea that in at least some of these stars the field is generated by an electric current generator could solve this paradox. Some of you may think of dynamos on bicycles: the rotation of a magnet produces an electric current. Here everything works the other way around: the movement of material leads to the emergence of electric currents, which, in turn, create a magnetic field.”
Professor Boris Gansike, University of Warwick
According to the proposed dynamo mechanism, the magnetic field is created by electric currents caused by convective motion in the core of the white dwarf. These convective currents are caused by the release of heat from the solidifying core. The main component of the dynamo is a solid core surrounded by a convective mantle, in the case of Earth, it is a solid iron core surrounded by convective liquid iron. A similar situation occurs with white dwarfs when they have cooled down enough.
Initially, after the star has ejected its envelope, the white dwarf is very hot and consists of liquid carbon and oxygen. However, when it cools enough, it begins to crystallize at the center, and the configuration becomes similar to that of the Earth: a solid core surrounded by convective fluid. Since the velocities in liquids can become much higher in white dwarfs than on Earth, the generated fields are potentially much stronger. This dynamo mechanism could explain the frequency of occurrence of highly magnetic white dwarfs in many different contexts, especially white dwarfs in binary stars.
The mechanism for generating a magnetic field is the same as on planets. This study explains how magnetic fields are generated in white dwarfs and why these magnetic fields are much stronger than on Earth. The next steps in this study will be the implementation of a more detailed model of the dynamo mechanism and experimental verification of additional predictions of this model.