When a star like our Sun reaches the end of its life, it can expand to ingest the surrounding planets and asteroids that were born with it. Using the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile, researchers, including Western physics and astronomy professor emeritus John Landstreet, have found a unique signature of this process for the first time, a scar imprinted on the surface of a white dwarf star.
The results are published today in The Astrophysical Journal Letters.
Landstreet, an expert on the evolutionary life history of stars, played a pivotal role in the discovery. His contributions were crucial in interpreting the data, particularly in understanding how the magnetic field affects the material distribution on the white dwarf star's surface.
John Landstreet (Contributed)
"We suspected that we should be able to observe a surface metal scar on a white dwarf star that has recently cannibalized old planet material - if the star has a magnetic field," said Landstreet. "We are now trying to understand better how the magnetic field controls the location and size of the scar."
Landsteet's insights contributed significantly to the broader understanding of stellar evolution, particularly in the context of white dwarfs.
"It is well known that some white dwarfs - slowly cooling embers of stars like our Sun - are cannibalizing 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," said Stefano Bagnulo, an astronomer at Armagh Observatory and Planetarium in Northern Ireland, U.K. and lead author of the study.
The scar the team observed is a concentration of metals imprinted on the surface of the white dwarf WD 0816-310, the Earth-sized remnant of a star that is similar to, but somewhat larger 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 second-largest asteroid in the Solar System," said Jay Farihi, a professor at University College London, U.K. and co-author on the study.
The observations also provided clues to how the star got its metal scar. The team noticed that the strength of the metal detection changed as the star rotated, 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 synchronized with changes in the white dwarf's magnetic field, indicating that this metal scar is located on one of its magnetic poles. Put together, these clues indicate that the magnetic field funnelled metals onto the star, creating the scar.
"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," said Landstreet, also a visiting astronomer at Armagh Observatory and Planetarium. "Nothing like this has been seen before."
To reach these conclusions, the team used a 'Swiss-army knife' instrument on the VLT called FORS2, which allowed them to detect the metal 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 sensitively measure stellar magnetic fields," said Bagnulo.
In their study, the team also relied on archival data from the VLT's X-shooter instrument to confirm their findings. instrument to confirm their findings.
Harnessing the power of observations like these, astronomers can reveal the bulk 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.'
"This project is part of a very large area of study. We are trying to understand how specific stars evolve," said Landstreet, emphasizing the importance of the work in piecing together the vast puzzle of the universe's life cycles.
Looking ahead, Landstreet and his collaborators aim to find more examples of this phenomenon to better understand the impact of magnetic fields on stellar evolution. Their work offers a glimpse into the complex processes that shape the life cycles of stars, contributing to the broader field of astronomy.
