There’s a lot we don’t understand about white dwarf stars, but a mystery may finally have a solution: how do some of these cosmic objects end up with insanely powerful magnetic fields?
According to new calculations and modeling, these super-dense objects may have a magnetosphere-forming dynamo – but the strongest white dwarf magnetic fields, a million times stronger than Earth’s, occur only in certain contexts.
Not only does research solve several long-standing problems, but it shows once again that very similar phenomena can be observed in wildly different astronomical objects, and that the universe is sometimes more like itself than we might have originally thought.
White dwarf stars are what we commonly call “dead”
The resulting object that shines brightly with residual thermal energy is a white dwarf and it is incredibly dense. Just a single teaspoon of white dwarf material would weigh about 15 tons, which means that it would not be unreasonable to assume that the interior of these objects would be very different from the interior of planets like Earth.
Astrophysicists have been trying to figure out how white dwarf stars can have powerful magnetic fields, at intervals up to about a million times stronger than Earth. In context, the sun’s magnetic field is twice as strong as the earth’s – so something unusual has to happen with white dwarfs.
However, it will be a bit difficult. Only some white dwarfs have powerful magnetic fields. White dwarfs in detached binaries – where no star exceeds the area of space within which stellar material is bound by gravity, known as a Roche patch – less than a billion years old do not have these magnetic fields.
But for white dwarfs in semi-detached binary areas, where one of the stars spills out of its Roche patch and the white dwarf weights gravity material from its accompanying companion, more than a third of these show strong magnetic fields. And a pair of highly magnetic white dwarfs also appear in older detached binaries.
Models from stellar evolution have not been able to explain how this happens, so an international team of astrophysicists took a different approach and proposed a nuclear dynamo that evolves over time rather than at the time of the formation of the white dwarf.
This dynamo would be a rotating, convection and electrically conductive fluid that converts kinetic energy into magnetic energy that spins a magnetic field into space. In the case of the earth, convection of liquid iron moving around the nucleus is generated.
“We have long known that something was missing in our understanding of magnetic fields in white dwarfs, as the statistics from observations simply did not make sense,” said physicist Boris Gänsicke of the University of Warwick in the United Kingdom.
“The idea that, at least in some of these stars, the field generated by a dynamo can solve this paradox.”
When a white dwarf is first formed, right after losing its outer envelope, it is very hot, consisting of liquid carbon and oxygen. According to the team model, heat escaping outward creates convection currents as the nucleus of the white dwarf cools and crystallizes, much like the way fluid moves around inside the earth, producing a dynamo.
“Since velocities in liquid can be much higher in white dwarfs than on Earth, the fields generated are potentially much stronger,” explained physicist Matthias Schreiber of the Federico Santa María University of Technology in Chile.
“This dynamo mechanism may explain the occurrence of highly magnetic white dwarfs in many different contexts, and especially those of white dwarfs in binary stars.”
As the white dwarf cools and gets older, its orbit with its binary companion gets closer. When the companion exceeds his Roche lap and the white dwarf begins to gather material, the white dwarf’s spin speed increases; this faster rotation also affects the dynamo and produces an even stronger magnetic field.
If this magnetic field is strong enough to connect the magnetic field of the binary companion, the binary companion exerts a torque that causes its orbital motion to synchronize with the spin of the white dwarf, which in turn causes the binary companion to detach from its Roche-lap. , returns the system to a detached binary. This process is repeated in the end.
Another mechanism is likely to be required to explain the strongest white dwarf magnetic field strengths, but for now, the team’s results are consistent with observations. White dwarfs in detached binaries are older than a billion years and have previously experienced mass transfer at a semi-detached stage interrupted when a wild magnetic field appeared.
If the team’s model is accurate, future white dwarf observations will continue to be consistent with their findings.
“The beauty of our idea is that the mechanism of magnetic field generation is the same as in planets,” Schreiber said.
“This research explains how magnetic fields are formed 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 only one area would have. had a hard time with. “
The research is published in Natural astronomy.