Researchers discover the mechanism thatlikely generates huge white dwarf magneticfields30 April 2021, by Peter Thorley

Illustration of the origin of magnetic fields in white dwarfsin close binaries (to be read counter clockwise). Themagnetic field appears when a crystallizing white dwarfaccretes from a companion star and as a consequencestarts to spin rapidly. When the white dwarfs fieldconnects with the field of the secondary star, masstransfer stops for a relatively short period of time. Credit:Paula Zorzi

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

One of the most striking phenomena inastrophysics is the presence of magnetic fields.Like the Earth, stars and stellar remnants such as white dwarfs have one. It is known that themagnetic fields of white dwarfs can be a milliontimes stronger than that of the Earth. However,their origin has been a mystery since the discoveryof the first magnetic white dwarf in the 1970s.Several theories have been proposed, but none ofthem has been able to explain the differentoccurrence rates of magnetic white dwarfs, both as

individual stars and in different binary starenvironments.

This uncertainty may be resolved thanks toresearch by an international team ofastrophysicists, including Professor Boris Gänsickefrom the University of Warwick and led byProfessor Dr. Matthias Schreiber from NúcleoMilenio de Formación Planetaria at UniversidadSanta María in Chile. The team showed that adynamo mechanism similar to the one thatgenerates magnetic fields on Earth and otherplanets can work in white dwarfs, and producemuch stronger fields. This research, part-funded bythe Science and Technology Facilities Council(STFC) and the Leverhulme Trust, has beenpublished in the prestigious scientific journal NatureAstronomy.

Professor Boris Gänsicke of the Department ofPhysics at the University of Warwick said: "Wehave known for a long time that there wassomething missing in our understanding ofmagnetic fields in white dwarfs, as the statisticsderived from the observations simply did not makesense. The idea that, at least in some of thesestars, the field is generated by a dynamo can solvethis paradox. Some of you may rememberdynamos on bicycles: turning a magnet produces electric current. Here, it works the other wayaround, the motion of material leads to electriccurrents, which in turn generate the magnetic field."

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A crystallizing magnetic white dwarf accreting from thewind of its companion star. Credit: Paula Zorzi

According to the proposed dynamo mechanism, themagnetic field is generated by electric currentscaused by convective motion in the core of thewhite dwarf. These convective currents are causedby heat escaping from the solidifying core.

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

The astrophysicist explains that at the beginning,after the star has ejected its envelope, the whitedwarf is very hot and composed of liquid carbonand oxygen. However, when it has sufficientlycooled, it begins to crystallize in the center and theconfiguration becomes similar to that of the Earth: asolid core surrounded by a convective liquid. "Asthe velocities in the liquid can become much higherin white dwarfs than on Earth, the generated fieldsare potentially much stronger. This dynamomechanism can explain the occurrence rates ofstrongly magnetic white dwarfs in many differentcontexts, and especially those of white dwarfs inbinary stars" he says.

Thus, this research could solve a decades-oldproblem. "The beauty of our idea is that themechanism of magnetic field generation is the

same as in planets. This research explains howmagnetic fields are generated in white dwarfs andwhy these magnetic fields are much stronger thanthose on Earth. I think it is a good example of howan interdisciplinary team can solve problems thatspecialists in only one area would have haddifficulty with," Schreiber adds.

The next steps in this research, says theastrophysicist, are to perform a more detailedmodel of the dynamo mechanism and to testobservationally the additional predictions of thismodel.

More information: Matthias R. Schreiber et al.The origin and evolution of magnetic white dwarfsin close binary stars, Nature Astronomy (2021). DOI: 10.1038/s41550-021-01346-8

Provided by University of Warwick

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APA citation: Researchers discover the mechanism that likely generates huge white dwarf magneticfields (2021, April 30) retrieved 31 August 2021 from https://phys.org/news/2021-04-mechanism-huge-white-dwarf-magnetic.html

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