The puzzle of white dwarfs having intense magnetic fields may finally be solved

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There is much we don’t understand about white dwarfs, but one mystery may finally be solved: How is it that some of these cosmic obje

There is much we don’t understand about white dwarfs, but one mystery may finally be solved: How is it that some of these cosmic objects have insanely strong magnetic fields?

New calculations and modeling suggest that these super-dense objects may have a dynamo that creates a magnetosphere – but white dwarfs’ strongest magnetic fields, a million times stronger than Earth’s, only occur in certain contexts.

The finding not only solves several long-standing problems, but shows once again that very similar phenomena can be observed in very different astronomical objects, and that the universe is sometimes more similar than we first think.

Dwarf white stars are what we colloquially refer to as “dead” stars. When a star less than eight times the mass of the Sun reaches the end of its lifespan because it has run out of elements suitable for nuclear fusion, it ejects its outer material. The remaining core collapses into an object less than 1.4 times the mass of the Sun, packed into a sphere about the size of Earth.

The resultant object, which glows brightly from the remaining thermal energy, is a white dwarf, and it is incredibly dense. 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 is very different from the interior of planets like Earth.

Attempts have been made by astrophysicists to figure out how white dwarf stars can have strong magnetic fields that are up to about a million times stronger than those of Earth. By comparison, the Sun’s magnetic field is twice as strong as Earth’s – so something unusual must be going on with white dwarfs.

It does get a little tricky, though. Only some white dwarfs have strong magnetic fields. White dwarfs in separated binaries – where neither star exceeds the region of space where stellar material is bound by gravity, known as the Roche lobe – that are less than a billion years old don’t have these magnetic fields.

But among white dwarfs in semi-separated binary systems, where one of the stars protrudes from its Roche lobe and the white dwarf’s gravity sucks up material from its lower-mass companion, more than a third of these stars have strong magnetic fields. And a few strongly magnetic white dwarfs also occur in older separated binary stars.

So a different approach was taken by an international team of astrophysicists, who proposed a nuclear dynamo that evolves over time rather than at the time of the white dwarf’s formation.

This dynamo would consist of a rotating, convective, and electrically conducting fluid that converts kinetic energy into magnetic energy and ejects a magnetic field into space. In Earth’s case, the convection is generated by liquid iron moving around the core.

“We’ve known for a long time that there was something missing in our understanding of magnetic fields in white dwarfs, because the statistics derived from observations just didn’t make sense,” says physicist Boris Gänsicke of the University of Warwick in the United Kingdom.

“The idea that for at least some of these stars, the field is generated by a dynamo can solve this paradox.”

When a white dwarf first forms, right after it loses its outer shell, it is very hot and made of liquid carbon and oxygen. As the core of the white dwarf cools and crystallizes, the heat escaping to the outside creates convection currents very similar to the way fluid moves inside the Earth, creating a dynamo, according to the team’s model.

“Because the velocities in the fluid in white dwarfs can get much higher 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 can explain the frequency of occurrence of strongly magnetic white dwarfs in many different contexts, especially white dwarfs in binary stars.”

As the white dwarf cools and ages, its orbit with its binary companion becomes narrower. As the companion exceeds its Roche lobe and the white dwarf begins to accrete material, the spin rate of the white dwarf increases; this faster rotation also affects the dynamo, creating an even stronger magnetic field.

When this magnetic field is strong enough to connect with the magnetic field of the binary companion, the binary companion exerts a torque that causes its orbital motion to align with the S

cts have insanely strong magnetic fields?

New calculations and modeling suggest that these super-dense objects may have a dynamo that creates a magnetosphere – but white dwarfs’ strongest magnetic fields, a million times stronger than Earth’s, only occur in certain contexts.

The finding not only solves several long-standing problems, but shows once again that very similar phenomena can be observed in very different astronomical objects, and that the universe is sometimes more similar than we first think.

Dwarf white stars are what we colloquially refer to as “dead” stars. When a star less than eight times the mass of the Sun reaches the end of its lifespan because it has run out of elements suitable for nuclear fusion, it ejects its outer material. The remaining core collapses into an object less than 1.4 times the mass of the Sun, packed into a sphere about the size of Earth.

The resultant object, which glows brightly from the remaining thermal energy, is a white dwarf, and it is incredibly dense. 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 is very different from the interior of planets like Earth.

Attempts have been made by astrophysicists to figure out how white dwarf stars can have strong magnetic fields that are up to about a million times stronger than those of Earth. By comparison, the Sun’s magnetic field is twice as strong as Earth’s – so something unusual must be going on with white dwarfs.

It does get a little tricky, though. Only some white dwarfs have strong magnetic fields. White dwarfs in separated binaries – where neither star exceeds the region of space where stellar material is bound by gravity, known as the Roche lobe – that are less than a billion years old don’t have these magnetic fields.

But among white dwarfs in semi-separated binary systems, where one of the stars protrudes from its Roche lobe and the white dwarf’s gravity sucks up material from its lower-mass companion, more than a third of these stars have strong magnetic fields. And a few strongly magnetic white dwarfs also occur in older separated binary stars.

So a different approach was taken by an international team of astrophysicists, who proposed a nuclear dynamo that evolves over time rather than at the time of the white dwarf’s formation.

This dynamo would consist of a rotating, convective, and electrically conducting fluid that converts kinetic energy into magnetic energy and ejects a magnetic field into space. In Earth’s case, the convection is generated by liquid iron moving around the core.

“We’ve known for a long time that there was something missing in our understanding of magnetic fields in white dwarfs, because the statistics derived from observations just didn’t make sense,” says physicist Boris Gänsicke of the University of Warwick in the United Kingdom.

“The idea that for at least some of these stars, the field is generated by a dynamo can solve this paradox.”

When a white dwarf first forms, right after it loses its outer shell, it is very hot and made of liquid carbon and oxygen. As the core of the white dwarf cools and crystallizes, the heat escaping to the outside creates convection currents very similar to the way fluid moves inside the Earth, creating a dynamo, according to the team’s model.

“Because the velocities in the fluid in white dwarfs can get much higher 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 can explain the frequency of occurrence of strongly magnetic white dwarfs in many different contexts, especially white dwarfs in binary stars.”

As the white dwarf cools and ages, its orbit with its binary companion becomes narrower. As the companion exceeds its Roche lobe and the white dwarf begins to accrete material, the spin rate of the white dwarf increases; this faster rotation also affects the dynamo, creating an even stronger magnetic field.

When this magnetic field is strong enough to connect with the magnetic field of the binary companion, the binary companion exerts a torque that causes its orbital motion to align with the S

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