White dwarfs have some surprisingly strong magnetic fields, and a team of astronomers may have finally figured out why. When they cool down, they can activate a dynamo mechanism similar to the Earth’s magnetic field.
Some white dwarfs have magnetic fields millions of times stronger than Earth, but the origins of those fields have been a mystery to astronomers since the discovery of the first magnetized white dwarf in the 1970s. The biggest problem is that white dwarfs have all sorts of different magnetic fields – and some don’t have them at all.
New research published in Nature Astronomy under the direction of Professor Dr. Matthias Schreiber from the Núcleo Milenio de Formación Planetaria at the Universidad Santa María in Chile could provide an answer: Dynamo processes in white dwarfs can excite some pretty impressive magnetic fields.
Professor Boris Gänsicke from the Department of Physics at Warwick University, who contributed to this work, says, “We have known for a long time that something is missing in our understanding of magnetic fields in white dwarfs, as the statistics show, the observations just made no 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 creates an electric current. Here it works the other way round, the movement of the material leads to electrical currents, which in turn generate the magnetic field. “
In order for a dynamo to work, you need a layer of convection material. The agitated, rotating material can pick up weak magnetic fields, fold them back on themselves and amplify them.
“The main component of the dynamo is a solid core that is surrounded by a convective jacket. In the case of the earth, it is a solid iron core surrounded by convective liquid iron. A similar situation occurs with white dwarfs when they have cooled down sufficiently, ”explains Matthias Schreiber.
When a white dwarf first forms, it’s a hot, dense ball of liquid carbon and oxygen. But when it cools down, some of the carbon and oxygen form a crystal lattice. This crystallized core forms the foundation on which a convective layer can sit. If the white dwarf accumulates material from a nearby companion, it can quickly spin and turn on the dynamo.
“Since the velocities in the liquid in white dwarfs can be much higher than on Earth, the fields generated are possibly much stronger. This dynamo mechanism can explain the occurrence rates of strongly magnetic white dwarfs in many different contexts, especially those of white dwarfs in binary stars, ”says Schreiber.
Dynamo mechanisms are common throughout the universe, and this work shows how these mechanisms can solve this decades-old problem. “The nice thing about our idea is that the mechanism by which magnetic fields are generated is the same as on planets. This research explains how magnetic fields are created in white dwarfs and why these magnetic fields are much stronger than those on Earth. I think it’s a good example of how an interdisciplinary team can solve problems that specialists in just one area would have had difficulties with, ”adds Schreiber.