The presence of magnetic fields is one of the most striking phenomena in astrophysics. Stars and stellar remnants such as white dwarfs, like the Earth, have one. It is known that white dwarfs’ magnetic fields can be a million times stronger than the Earth’s. Their origin, however, has remained a mystery since the discovery of the first magnetic white dwarf in the 1970s. Several theories have been proposed, but none have been able to explain the different occurrence rates of magnetic white dwarfs as individual stars or in different binary star environments.
This uncertainty may be resolved thanks to research by an international team of astrophysicists led by Professor Dr. Matthias Schreiber of the Ncleo Milenio de Formación Planetaria at Universidad Santa Mara in Chile, including Professor Boris Gänsicke from the University of Warwick. The researchers demonstrated that a dynamo mechanism similar to the one that generates magnetic fields on Earth and other planets can be used to generate much stronger fields in white dwarfs. The findings of this study, which were funded in part by the Science and Technology Facilities Council (STFC) and the Leverhulme Trust, were published in the prestigious scientific journal Nature Astronomy.
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.
Professor Boris Gänsicke of the University of Warwick’s Department of Physics stated: “We’d known for a long time that there was something missing in our understanding of magnetic fields in white dwarfs because the statistics derived from observations simply didn’t make sense. This paradox can be solved by the idea that, at least in some of these stars, the field is generated by a dynamo. Some of you may recall dynamos on bicycles, which produced an electric current by turning a magnet. In this case, the motion of the material generates electric currents, which in turn generate the magnetic field.”
The magnetic field is generated by electric currents caused by convective motion in the core of the white dwarf, according to the proposed dynamo mechanism. Heat escaping from the solidifying core causes these convective currents.
“The dynamo’s main component is a solid core surrounded by a convective mantle—in the case of the Earth, this is a solid iron core surrounded by convective liquid iron. When white dwarfs have sufficiently cooled, they experience a similar situation “Matthias Schreiber explains.
Magnetic white dwarfs with isolated fields have field strengths ranging from kilogauss to gigagauss. However, the magnetic field’s origin has yet to be determined. It is unknown whether these fields are fossil, i.e., the remnants of original weak magnetic fields amplified during the evolution of their progenitor stars, or the result of binary interactions, or, finally, produced by other internal physical mechanisms during the cooling of the white dwarf itself.
According to the astrophysicist, at first, after the star has ejected its envelope, the white dwarf is extremely hot and composed of liquid carbon and oxygen. However, once sufficiently cooled, it begins to crystallize in the center, resulting in a configuration similar to that of the Earth: a solid core surrounded by a convective liquid. “Because the liquid velocities in white dwarfs can reach much higher speeds than on Earth, the generated fields have the potential to be much stronger. This dynamo mechanism can explain the occurrence rates of strongly magnetic white dwarfs in a variety of settings, particularly white dwarfs in binary stars “he claims.
As a result, this study has the potential to solve a decades-old problem. “The genius of our concept is that the mechanism of magnetic field generation is identical to that of planets. This study explains how magnetic fields are generated in white dwarfs and why they are much stronger than those found on Earth. It is, in my opinion, a good example of how an interdisciplinary team can solve problems that specialists in only one field would struggle with “Schreiber continues.
According to the astrophysicist, the next steps in this research will be to perform a more detailed model of the dynamo mechanism and to test the additional predictions of this model through observation.
