This cannibalistic white dwarf has scars from the planets it eats

A magnetic white dwarf devours planets and asteroids, leaving metallic traces on its surface

When a star like our Sun reaches the end of its life, it can devour the surrounding planets and asteroids that were born with it. Thanks to the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile, researchers have for the first time found a unique signature of this process: a scar left on the surface of a white dwarf star. The results are published in The Astrophysical Journal Letters.

The fate of a star after its death depends on its size. The largest become black holes, but those like our sun do not collapse completely. “Some white dwarfs – remnants of slowly cooling stars like our Sun – are known to cannibalize parts of their planetary systems. “We have now found that the star’s magnetic field plays a key role in this process, leading to a scar on the surface of the white dwarf,” says Stefano Bagnulo, astronomer at the Armagh Observatory and Planetarium in Northern Ireland, UK. United) and lead author of the study.

The scar the team observed is a concentration of metals imprinted on the surface of white dwarf WD 0816-310, the remnant of the Earth-sized star that is similar to our sun but slightly larger. “We have shown that these metals come from a planetary fragment the same size or possibly larger than Vesta, which is about 500 kilometers in diameter and is the second largest asteroid in the solar system,” says Jay Farihi, a professor at University College London. United Kingdom. United Kingdom) and co-author of the study.

A metallic scar

The observations also provided clues about how the star got its metallic scar. The team observed that the intensity of metal detection changed with the star’s rotation, suggesting that metals are concentrated in a specific area of ​​the white dwarf’s surface rather than evenly distributed across it. They also found that these changes were synchronized with changes in the white dwarf’s magnetic field, suggesting that this metallic scar is located at one of its magnetic poles. Taken together, these clues suggest that the magnetic field funneled metals into the star, creating the scar.

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“Surprisingly, the material was not evenly mixed across the star’s surface, as theory predicted. “Instead, this scar is a concentrated patch of planetary material held in place by the same magnetic field that guided the bending fragments,” says co-author John Landstreet, a professor at Western University in Canada, also a partner of the Armagh Observatory and Planetarium. “Nothing like this had ever been seen before.”

To reach these conclusions, the team at VLT used a “Swiss Army knife” instrument called FORS2, which allowed them to detect the metallic scar and connect it to the star’s magnetic field. “ESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs and to sensitively measure stellar magnetic fields,” says Bagnulo. In their study, the team also relied on archival data from the VLT’s X-Shooter instrument to confirm their findings.

By harnessing the power of such observations, astronomers can reveal the composition of exoplanets, planets orbiting other stars outside the solar system. This unique study also shows how planetary systems can remain dynamically active even after “death”.

Astronomers had previously observed numerous white dwarfs contaminated by metals spread across the star’s surface. These metals are known to come from decayed planets or asteroids that get too close to the star, following stellar orbits similar to those of comets in our solar system. However, in the case of WD 0816-310, the team is confident that the vaporized material was ionized and guided to the magnetic poles by the white dwarf’s magnetic field. The process has similarities to the formation of auroras on Earth and Jupiter.


Discovery of magnetically guided metal deposition on a polluted white dwarf

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