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Metal Scar Found on Cannibal Star: A Unique Discovery

A stronomers utilizing the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile have identified a distinct scar on the surface of a white dwarf star, shedding light on the cannibalistic nature of aging stars. The findings, detailed in The Astrophysical Journal Letters, mark the first time such a phenomenon has been observed.


Lead author of the study, Stefano Bagnulo, an astronomer at Armagh Observatory and Planetarium in Northern Ireland, UK, comments, "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."


The scar observed by the team is a concentration of metals imprinted on the surface of the white dwarf WD 0816-310, an Earth-sized remnant of a star akin to our sun. Jay Farihi, a professor at University College London, UK, and co-author on the study, notes, "We have demonstrated that these metals originate from a planetary fragment as large as or possibly larger than Vesta, which is about 500 kilometers across and the second-largest asteroid in the solar system."


Crucially, the researchers observed changes in metal detection strength as the star rotated, indicating concentration on a specific area rather than a uniform spread. These changes correlated with shifts in the white dwarf's magnetic field, suggesting that the scar is situated on one of its magnetic poles. This implies that the magnetic field funneled metals onto the star, creating the scar—a process akin to the formation of auroras on Earth and Jupiter.


John Landstreet, a professor at Western University, Canada, and co-author, highlights the unprecedented nature of this discovery, stating, "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. Nothing like this has been seen before."


The team employed the FORS2 instrument on the VLT, often likened to a "Swiss-army knife," to detect and connect the metal scar to the star's magnetic field. Additionally, archival data from the VLT's X-shooter instrument confirmed their findings.


Bagnulo emphasizes the unique capabilities of ESO in facilitating such observations, stating, "ESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs and sensitively measure stellar magnetic fields."

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