Astronomers have made an intriguing discovery, spotting a star with a dark metallic “scar” on its surface. This scar is believed to be the imprint of a doomed planetary fragment that came too close to its host. The star in question is a white dwarf called WD 0816-310, located about 63 light years away from Earth. It is a dense, Earth-sized remnant of a star that would have been similar to our sun during its lifetime.
The observations of WD 0816-310 revealed a concentrated patch of metals on its surface, which scientists believe to be the remnants of an ingested chunk of planet or an asteroid. “It is well known that some white dwarves – slowly cooling embers of stars like our Sun – are cannibalizing pieces of their planetary systems,” explains Stefano Bagnulo, an astronomer at the Armagh observatory and planetarium in Northern Ireland, UK, and lead author of the study.
Previously, it was assumed that these fragments would sink into the star and become evenly distributed over its surface. However, the latest findings challenge this assumption. The planetary material appears to be locked in place by the star’s magnetic field, resulting in a shadowy surface structure. Interestingly, the metallic patch covers a larger fraction of the pole than the equivalent of Antarctica on Earth.
White dwarfs are the stellar cores left behind after dying stars have exhausted their nuclear fuel and expelled their outer layers to form a planetary nebula. These remnants are roughly the size of Earth but incredibly dense. Their gravity is powerful enough to rip apart planets or asteroids that come too close.
Professor Jay Farihi from University College London, who co-authored the study, explains that when a planet or asteroid gets close to a white dwarf, it is tidally shredded. This process represents the final death spiral of these fragments. Farihi adds, “We’ve never seen this before. We now know the last moments before it’s gobbled up.”
Calculations suggest that the metals detected on WD 0816-310’s surface must have originated from a planetary fragment as large as, or possibly larger than, Vesta. Vesta is the second-largest asteroid in our solar system, measuring about 500km across.
The researchers also noticed that the strength of the metal detection changed as the star rotated. This indicates that the metals are concentrated on a specific area of the white dwarf’s surface rather than being evenly spread. Furthermore, these changes were synchronized with changes in the white dwarf’s magnetic field, suggesting that the metal scar is located on one of its magnetic poles. This information leads scientists to believe that the magnetic field funneled metals onto the star, creating the scar.
According to Professor John Landstreet from the University of Western Ontario and the Armagh observatory and planetarium, this scar represents a concentrated patch of planetary material held in place by the same magnetic field that guided the infalling fragments. He notes, “Nothing like this has been seen before.”
Astronomers are particularly interested in observing polluted white dwarfs because analyzing the metals and elements present can provide unprecedented insights into the bulk composition of exoplanets. These findings have been published in the Astrophysical Journal Letters.
The discovery of WD 0816-310 and its dark metallic scar opens up new possibilities for understanding the dynamics of white dwarfs and their interactions with planetary systems. It sheds light on the fate of planetary fragments that come too close to these stellar remnants and provides valuable information about the composition of exoplanets. As astronomers continue to explore the vastness of space, they uncover more mysteries and gain a deeper understanding of the universe we inhabit.
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