There are many things we don’t understand about white dwarf stars, but one mystery may finally have a solution: how do some of these space objects end up with insanely powerful magnetic fields?
According to new calculations and modeling, these superdense objects may have a magnetosphere-generating dynamo – but the strongest white dwarf magnetic fields, a million times more powerful than Earth’s, occur only in a certain context.
The study not only solves several long-standing problems, but once again shows that many similar phenomena can be observed in wildly different astronomical objects, and that sometimes the universe is more like itself than we might initially think.
White dwarf stars are what we colloquially call “dead”
The resulting object, shining brightly with residual heat, is a white dwarf and is incredibly dense. Just one teaspoon of white dwarf material will weigh about 15 tons, which means that it would not be unreasonable to assume that the interior of these objects will be very different from the interior of planets like Earth.
Astrophysicists are trying to figure out how white dwarfs can have powerful magnetic fields, in ranges up to about a million times stronger than Earth’s. In context, the Sun’s magnetic field is twice as powerful as the Earth’s – so something unusual must happen to white dwarfs.
However, it becomes a bit complicated. Only some white dwarfs have powerful magnetic fields. White dwarfs in separate binary files – in which no star exceeds the area of space in which stellar material is bound by gravity, known as the Roche lobe – do not have these magnetic fields for less than a billion years.
But for white dwarfs in semi-detached binaries, where one of the stars spills out of its Roche lobe and the white dwarf gravitational ejects material from its lower-mass satellite, more than a third of them show strong magnetic fields. And a few highly magnetic white dwarfs also appear in older stand-alone binaries.
Models of stellar evolution are unable to explain how this happens, so an international team of astrophysicists has taken a different approach, offering a basic dynamo that evolves over time rather than during the formation of the white dwarf.
This dynamo would be a rotating, convective, and electrically conductive fluid that converts kinetic energy into magnetic energy by rotating a magnetic field out into space. In the case of the Earth, convection is generated by liquid iron moving around the core.
“We have long known that there is something missing in our understanding of magnetic fields in white dwarfs, because the statistics obtained from observations simply do not make sense,” said physicist Boris Gensike of the University of Warwick in the UK.
“The idea that, at least for some of these stars, the field is generated by a dynamo may solve this paradox.”
When the white dwarf first forms, as soon as it loses its outer shell, it is very hot, made up of liquid carbon and oxygen. According to the team’s model, as the white dwarf’s core cools and crystallizes, the heat coming out creates convective currents, much like the way liquid moves inside the Earth, producing a dynamo.
“Because velocities in liquids can become much higher in white dwarfs than on Earth, the generated fields are potentially much stronger,” explained physicist Matthias Schreiber of the Federico Santa Maria Technical University in Chile.
“This dynamo mechanism can explain the frequency of highly magnetic white dwarfs in many different contexts, and especially those of white dwarfs in binary stars.”
As the white dwarf cools and ages, its orbit with its binary satellite approaches. When the satellite exceeds its Roche lobe and the white dwarf begins to accumulate material, the speed of rotation of the white dwarf increases; this faster rotation also affects the dynamo, creating an even stronger magnetic field.
If this magnetic field is strong enough to connect with the magnetic field of the binary satellite, the binary satellite exerts torque that causes its orbital motion to synchronize with the rotation of the white dwarf, which in turn causes the binary satellite to separate from its Roche lobe, returning the system to a separate binary file. This process will eventually be repeated.
A different mechanism will probably be needed to explain the strongest strength of the white dwarf’s magnetic field, but so far the team’s results are in line with observations. White dwarfs in separate binary files are over a billion years old and have previously undergone mass transference in a semi-detached phase, interrupted when a wild magnetic field appears.
If the team model is accurate, future observations of white dwarfs will continue to be in line with their findings.
“The beauty of our idea is that the mechanism for generating a magnetic field is the same as on planets,” Schreiber said.
“This study explains how magnetic fields are generated 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 experts in only one field would had difficulty with. “
The study was published in Natural astronomy.