There’s lots we do not perceive about white dwarf stars, however one thriller might lastly have an answer: how do a few of these cosmic objects find yourself having insanely highly effective magnetic fields?
According to new calculations and modelling, these super-dense objects can have a magnetosphere-generating dynamo – however the strongest white dwarf magnetic fields, one million instances extra highly effective than Earth’s, solely happen inside sure contexts.
Not solely does the analysis resolve a number of long-standing issues, however as soon as once more it exhibits that very comparable phenomena may be noticed in wildly completely different astronomical objects, and that typically the Universe is extra like itself than we would initially assume.
White dwarf stars are what we colloquially name “dead” stars. When a star lower than round eight instances the mass of the Sun reaches the top of its lifespan, having run out of components appropriate for nuclear fusion, it ejects its outer materials. The remaining core collapses down into an object lower than 1.4 instances the mass of the Sun, packed right into a sphere across the dimension of Earth.
The ensuing object, shining brightly with residual thermal vitality, is a white dwarf, and it is extremely dense. Just a single teaspoon of white dwarf materials would weigh round 15 tons, which suggests it might not be unreasonable to imagine that the interiors of those objects could be very completely different from the interiors of planets like Earth.
Astrophysicists have been making an attempt to work out how white dwarf stars can have highly effective magnetic fields, in ranges as much as round one million instances stronger than Earth’s. For context, the Sun’s magnetic area is twice as highly effective as Earth’s – so one thing uncommon must be occurring with white dwarfs.
It will get a little bit tough, although. Only some white dwarfs have highly effective magnetic fields. White dwarfs in indifferent binaries – by which neither star exceeds the area of house inside which stellar materials is sure by gravity, referred to as a Roche lobe – lower than a billion years outdated do not have these magnetic fields.
But for white dwarfs in semi-detached binaries, the place one of many stars spills out of its Roche lobe, and the white dwarf is gravitationally slurping materials off its lower-mass companion, greater than a 3rd of those exhibit sturdy magnetic fields. And a number of strongly magnetic white dwarfs seem in older indifferent binaries, too.
Stellar evolution fashions have been unable to clarify how this occurs, so a global crew of astrophysicists took a distinct strategy, proposing a core dynamo that develops over time, relatively than on the time of the white dwarf’s formation.
That dynamo could be a rotating, convecting, and electrically conducting fluid that converts kinetic vitality into magnetic vitality, spinning a magnetic area out into house. In Earth’s case, convection is generated by liquid iron transferring across the core.
“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,” said physicist Boris Gänsicke of the University of Warwick within the UK.
“The idea that, at least in some of these stars, the field is generated by a dynamo can solve this paradox.”
When a white dwarf first kinds, proper after dropping its outer envelope, it is extremely popular, made up of fluid carbon and oxygen. According to the crew’s mannequin, because the core of the white dwarf cools and crystallizes, warmth escaping outwards creates convection currents, similar to the way in which fluid strikes round inside Earth, producing a dynamo.
“As the velocities in the liquid 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 María Technical University in Chile.
“This dynamo mechanism can explain the occurrence rates of strongly 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 companion grows nearer. When the companion exceeds its Roche lobe, and the white dwarf begins accreting materials, the spin charge of the white dwarf will increase; this quicker rotation additionally impacts the dynamo, producing a good stronger magnetic area.
If this magnetic area is powerful sufficient to attach with the magnetic area of the binary companion, the binary companion exerts a torque that causes its orbital movement to synchronize with the white dwarf’s spin, which in flip causes the binary companion to detach from its Roche lobe, returning the system to a indifferent binary. This course of will ultimately repeat.
A special mechanism will most likely be required to clarify the very strongest white dwarf magnetic area strengths, however for now, the crew’s outcomes are per observations. White dwarfs in indifferent binaries are older than a billion years, and have beforehand skilled mass switch in a semi-detached stage lower quick when a wild magnetic area appeared.
If the crew’s mannequin is correct, future white dwarf observations will proceed to be per 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 generated in white dwarfs and why these magnetic fields are much stronger than those on Earth. I think it is a good example of how an interdisciplinary team can solve problems that specialists in only one area would have had difficulty with.”
The analysis has been revealed in Nature Astronomy.