Astronomers observe a white dwarf star with a metallic “scar”. space

Astronomers have discovered a star with a dark metallic “scar” on its surface, believed to be the imprint of a doomed planetary fragment that got too close to its host.

The white dwarf star, called WD 0816-310, is the dense, Earth-sized remnant of a star about 63 light-years away that would have been similar to our Sun in its lifetime. Observations revealed a concentrated patch of minerals on its surface, which appears to be the remains of a piece of an swallowed planet or asteroid.

“Some white dwarfs — which slowly cool the embers of stars like our Sun — are known to break up parts of their planetary systems,” said Stefano Pagnolo, an astronomer at Armagh Observatory and Planetarium in Northern Ireland, UK, and lead author of the book. the study.

Previously, it was assumed that these fragments would sink into the star and be evenly distributed over the surface. However, the latest results reveal that planetary material is apparently locked in place by the star's magnetic field, resulting in a mysterious surface structure. The mineral patch covers a larger portion of the pole than the Earth's equivalent of Antarctica.

A white dwarf is the core of a star left behind after a dying star has exhausted its nuclear fuel and sheds its outer layers to form a glowing cloud, known as a planetary nebula. Roughly the size of Earth, it is so dense that its gravity can tear apart planets or asteroids that come into its vicinity.

“When a planet or asteroid approaches, it gets torn apart by the tides,” said Guy Farrehy, professor of astrophysics at University College London and co-author of the study. This is the ultimate death spiral [of one of these fragments]. We have never seen this before. We now know the last moments before they are devoured.

Calculations indicate that the minerals detected on the surface of the star must have originated from a large planetary fragment such as Vesta or perhaps larger, which is about 500 kilometers across, and is the second largest asteroid in the solar system.

The team observed that the strength of the metal detection changes as the star rotates, indicating that the metals are concentrated in a specific area on the white dwarf's surface, rather than spread smoothly across it.

They also found that these changes were concurrent with changes in the white dwarf's magnetic field, suggesting that this mineral scar is located on one of its magnetic poles. Together, this evidence suggests that the magnetic field directed metals into the star, creating the scar.

Professor John Landstreet, from the University of Western Ontario in Canada and the Armagh Observatory and Planetarium, said: “This scar is a concentrated patch of planetary material, held in place by the same magnetic field that directed the falling fragments. Nothing like this has been seen before.”

Astronomers watch so-called polluted white dwarfs because measuring the metals and elements present can give unprecedented insights into the bulk composition of exoplanets.

The results are published in Astrophysical Journal Letters.