Zombie star earned metal scar by devouring its own planets

A dead star devouring its own planets like a space zombie has been scarred by planetary debris.



The metallic scar on the surface of the white dwarf, designated WD 0816-310, was spotted by a team of astronomers using the Very Large Telescope (VLT) in the Atacama Desert in northern Chile. The “scar” represents the first unique mark on a white dwarf left by devoured planets and asteroids.


It is well known that some white dwarfs, the slowly cooling remains of stars like the Sun, eat parts of their own planetary systems. We now know that the magnetic field of the stars plays a key role in this process, leaving a scar on the surface of the white dwarf.



— Stefano Bagnulo, team leader and astronomer at Armagh Observatory


A team of astronomers was able to use VLT to determine the origin of the metal scar on WD 0816-310.


We showed that this metal comes from a fragment the size of Vesta or larger, the second largest asteroid in the solar system with a diameter of 500 km.


— Jay Farihi, discovery team member and professor at University College London


White dwarfs form when stars the size of the sun run out of fuel needed for nuclear fusion in their cores. As soon as this process ends, the outgoing flow of energy compensating for gravity, which has supported the star for billions of years, also ends.


This leads to the collapse of the star’s core, while the outer layers, where nuclear reactions took place, swell tens to hundreds of times compared to the original size of the dying star. This is the red giant phase that the Sun will experience in about 5 billion years. Once it becomes a red giant, it will inflate to the orbit of Mars and engulf the inner planets, including Earth.


The red giant phase is short – the outer layers continue to dissipate and cool. As a result, what remains is a planetary nebula of gas and dust, consisting of stellar matter that surrounded the smoldering stellar core, which turned into a cosmic ember – a white dwarf.


These white dwarfs can then begin to absorb the remaining bodies in the systems, such as planets and asteroids, that were not destroyed during the red giant stage.


Astronomers have already observed white dwarfs “contaminated” with metals from absorbed bodies. This contamination was thought to occur when objects come too close to these dead stars.


However, the team’s new observation is different in that the accumulation of substance on the surface of WD 0816-310 resembles a scar concentrated in one place.


The intensity of the signal from the metals in WD 0816-310 appears to change as the dead star rotates. This means that the metals are concentrated in a specific location rather than scattered throughout the surface. Moreover, the observed changes are synchronized with fluctuations in the white dwarf’s magnetic field, which suggests that its metallic scar is located at one of the poles.


Surprisingly, contrary to theoretical predictions, the matter was not evenly distributed over the surface of the star. Instead, the scar is a concentrated patch of planetary material, held in place by the same magnetic field that guided the falling fragments. Nothing like this had ever been seen before.


— Western University Professor John Landstreet


To reach these conclusions, the team used a VLT tool called Focal Reducer and low dispersion Spectrograph (FORS2). FORS2 has been described as the “Swiss knife” of Paranal Observatory instruments due to its ability to study many different astronomical objects in many different ways.


FORS2 was sensitive enough for the team to observe this faint white dwarf and measure its magnetic field. This is what led to the identification of the scar.


In addition to demonstrating that planetary systems remain dynamically active after the death of a star, the results highlight the power of FORS2 in studying the overall composition of exoplanets.


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