Solution Proposed to Mystery of Incredibly Strong White Dwarf Magnetic Fields

Illustration of the origin of magnetic fields in white dwarfs in close binaries (to be read counter clockwise). The magnetic field appears when a crystallizing white dwarf accretes from a companion star and as a consequence starts to spin rapidly. When the white dwarfs field connects with the field of the secondary star, mass transfer stops for a relatively short period of time.
Credit: Paula Zorzi
  • University of Warwick astronomer co-authors new study in Nature Astronomy that proposes a solution to long-running question of how white dwarf stars generate magnetic fields

  • A dynamo mechanism, similar to how Earth generates its magnetic field, could be the answer

  • This research shows that sometimes very similar mechanisms can work in very different astronomical objects.

A dynamo mechanism could explain the incredibly strong magnetic fields in white dwarf stars according to an international team of scientists, including a University of Warwick astronomer.

One of the most striking phenomena in astrophysics is the presence of magnetic fields. Like the Earth, stars and stellar remnants such as white dwarfs have one. It is known that the magnetic fields of white dwarfs can be a million times stronger than that of the Earth. However, their origin has been 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 occurrence rates of magnetic white dwarfs, both as individual stars and in different binary star environments.

This uncertainty may be resolved thanks to research by an international team of astrophysicists, including Professor Boris Gänsicke from the University of Warwick and led by Professor Dr. Matthias Schreiber from Núcleo Milenio de Formación Planetaria at Universidad Santa María in Chile. The team showed that a dynamo mechanism similar to the one that generates magnetic fields on Earth and other planets can work in white dwarfs, and produce much stronger fields. This research, part-funded by the Science and Technology Facilities Council (STFC) and the Leverhulme Trust, has been published in the prestigious scientific journal Nature Astronomy.

Professor Boris Gänsicke of the Department of Physics at the University of Warwick said: “We have known for a long time that there was something missing in our understanding of magnetic fields in white dwarfs, as the statistics derived from the observations simply did not make sense. The idea that, at least in some of these stars, the field is generated by a dynamo can solve this paradox. Some of you may remember dynamos on bicycles: turning a magnet produces electric current. Here, it works the other way around, the motion of material leads to electric currents, which in turn generate the magnetic field.”

According to the proposed dynamo mechanism, the magnetic field is generated by electric currents caused by convective motion in the core of the white dwarf. These convective currents are caused by heat escaping from the solidifying core.

“The main ingredient of the dynamo is a solid core surrounded by a convective mantle — in the case of the Earth, it is a solid iron core surrounded by convective liquid iron. A similar situation occurs in white dwarfs when they have cooled sufficiently,” explains Matthias Schreiber.

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