Scientists propose reason for strong white dwarf magnetic fields

An international team of scientists has proposed the reason for incredibly strong magnetic fields in white dwarf stars.

A new study by the scientists – The Origin and Evolution of Magnetic White Dwarfs in Close Binary Stars – published in the journal Nature Astronomy states a dynamo mechanism could be the reason.

Earth generates its magnetic field through a similar mechanism called geodynamo.

The first magnetic white dwarf was discovered in 1970s. The origin of white dwarf magnetic fields has been a mystery since then. Although scientists have proposed several theories, none offers explanation for the different occurrence rates of both individual magnetic white dwarfs and those that are in different binary star systems.

Professor Dr. Matthias Schreiber from Núcleo Milenio de Formación Planetaria at Universidad Santa María in Chile led the research.

One of the research team’s scientists, professor Boris Gänsicke of the department of physics at the University of Warwick stated, “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.”

He added, “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.”

The scientists proposed that the white dwarf magnetic field is generated by electric currents produced by convective motion in the white dwarf’s core. Heat coming from the solidifying core causes these convective currents.

“Convection is the circular motion that happens when warmer air or liquid – which has faster moving molecules, making it less dense – rises, while the cooler air or liquid drops down,” according to vocabulary.com. “Convection is a major factor in weather. The sun heats the earth’s surface, then, when cooler air comes into contact with it, the air warms and rises, creating an upward current in the atmosphere. That current can result in wind, clouds, or other weather.

“Convection currents within the earth move layers of magma, and convection in the ocean creates currents. Convection ovens use fans to circulate heat so that whatever you’re cooking cooks faster and more evenly.”

Schreiber said, “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,” adding, “A similar situation occurs in white dwarfs when they have cooled sufficiently.”