Free Astronomy Magazine May-June 2024

31 MAY-JUNE 2024 ASTRO PUBLISHING W hen a star like our Sun reaches the end of its life, it can ingest the sur- rounding planets and asteroids that were born with it. Now, using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile, researchers have found a unique signature of this process for the first time — a scar imprinted on the surface of a white dwarf star. “It is well known that some white dwarfs — slowly cooling embers of stars like our Sun — are cannibalis- ing pieces of their planetary systems. Now we have discovered that the star’s magnetic field plays a key role in this process, resulting in a scar on the white dwarf’s surface,” says Stefano Bagnulo, an astronomer at Armagh Observatory and Planetarium in Northern Ire- land, UK, and lead author of the study. The scar the team observed is a concentration of metals im- printed on the surface of the white dwarf WD 0816-310, the Earth-sized remnant of a star similar to, but somewhat more massive than, our Sun. “We have demonstrated that these metals originate from a planetary fragment as large as or possibly larger than Vesta, which is about 500 kilometres across and the sec- “Surprisingly, the material was not evenly mixed over the surface of the star, as predicted by theory. Instead, this scar is a concentrated patch of planetary material, held in place by the same magnetic field that has guided the infalling fragments,” says co-author John Landstreet, a professor at Western University, Canada, who is also affiliated with the Armagh Observatory and Plane- tarium. “Nothing like this has been seen before.” To reach these conclu- sions, the team used a ‘Swiss-army knife’ instrument on the VLT called FORS2, which allowed them to de- tect the metal scar and connect it to the star’s magnetic field. “ESO has the unique combination of capabili- ties needed to observe faint objects such as white dwarfs, and sensitively measure stellar magnetic fields,” says Bag- nulo. In their study, the team also relied on archival data from the VLT’s X-shooter instrument to confirm their findings. Harnessing the power of observations like these, astronomers can re- veal the bulk composition of exoplanets, planets orbiting other stars outside the Solar System. This unique study also shows how planetary sys- tems can remain dynamically active, even after ‘death’. ! he background artist’s impression shows the mag- netic white dwarf WD 0816-310, where astronomers have found a scar imprinted on its surface as a result of having ingested planetary debris. The video above sum- marises the discovery. [ESO/L. Calçada] ond-largest asteroid in the Solar Sys- tem,” says Jay Farihi, a professor at University College London, UK, and co-author on the study. The observa- tions also provided clues to how the star got its metal scar. The team no- ticed that the strength of the metal detection changed as the star ro- tated, suggesting that the metals are concentrated on a specific area on the white dwarf’s surface, rather than smoothly spread across it. They also found that these changes were synchronised with changes in the white dwarf’s magnetic field, indi- cating that this metal scar is located on one of its magnetic poles. Put to- gether, these clues indicate that the magnetic field funneled metals onto the star, creating the scar.