Metal scar discovered on cannibal star

When a star like our Sun reaches the end of its existence, it can swallow the planets and asteroids surrounding it. Using the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile, researchers have now discovered for the first time a clear trace of this process: a scar left on the surface of a white dwarf star. This result was published today in The Astrophysical Journal Letters.

‘It is well known that some white dwarfs – the slowly cooling remnants of stars such as our Sun – are eating parts of their planetary systems. We have now discovered that the star’s magnetic field plays a crucial role in this, resulting in a scar on the white dwarf’s surface,” says Stefano Bagnulo, astronomer at Armagh Observatory and Planetarium in Northern Ireland (UK) and lead author of the research.

The scar the team observed is a concentration of metals on the surface of white dwarf WD 0816-310 – the Earth-sized remnant of a star that was similar to our Sun, but slightly larger. “We showed that these metals came from a planetary debris at least the size of Vesta, which at about 500 kilometers in diameter is the second largest asteroid in our solar system,” said Jay Farihi, professor at University College London (UK) and co-author of the study.

The observations have also provided clues as to how the star acquired its metal scar. The team found that the metal detection strength varied as the star rotated. This suggested that the metals have accumulated on a specific part of the white dwarf’s surface, rather than being evenly distributed across its surface. The researchers also found that these variations were in sync with changes in the white dwarf’s magnetic field, indicating that the metal scar is located at one of its magnetic poles. Taken together, these clues indicate that the magnetic field has funneled metals into the star, causing the scar [1].

‘Surprisingly, the material was not evenly distributed over the star’s surface, as theory predicted. Instead, a build-up of planetary material has emerged, held in place by the same magnetic field that guided the crashing debris to the star’s surface,” said co-author John Landstreet, a professor at Western University in Canada, who is also affiliated with the Armagh Observatory and Planetarium. “Nothing like this has ever been seen before.”

To reach these conclusions, the team used FORS2, a versatile instrument on the VLT that allowed them to detect the metal scar and relate it to the star’s magnetic field. “ESO has the unique combination of tools needed to observe white dwarfs and accurately measure their magnetic fields,” Bagnulo said. The team also used archival data from the VLT’s X-shooter instrument to confirm their findings.

Observations like these allow astronomers to determine the global composition of exoplanets – planets that orbit stars outside the solar system. This unique research also shows that planetary systems can remain dynamically active, even after their ‘death’.

Nuts

[1] Astronomers have previously observed many white dwarfs contaminated with metals spread across the star’s surface. These metals are known to come from destroyed planets or asteroids that came too close to the star, following paths similar to those of our solar system’s comets. In the case of WD 0816-310, however, the team is confident that vaporized material was ionized and guided toward the magnetic poles by the white dwarf’s magnetic field. This process is similar to the way the auroras are formed on Earth and Jupiter.

Source: ESO