1st-ever metal scar on a cannibal star

Astronomers using the Very Large Telescope of the European Southern Observatory (ESO) have found a metal “scar” on the surface of a white dwarf star. This video – by ESO – summarizes the discovery.

  • When sunlike stars die, they swell into huge red giant stars. Later, they collapse to become small, dense white dwarf stars. In this process of swelling then collapsing, they can consume or “cannibalize” their planets. Astronomers have seen metals scattered on white dwarf surfaces, presumably from long-gone planets.
  • Now, for the first time, astronomers have identified a concentrated patch or scar on a white dwarf called WD 0816-310.
  • The scar on WD 0816-310 is located on one of its magnetic poles, suggesting that its magnetic field played a crucial role in the process during which the white dwarf “ate” its planet.

Metal scar on a cannibal star

Astronomers said on February 26, 2024, that they’ve found a “metal” scar on a cannibal white dwarf star for the first time.

Prior to this discovery, they had found “metals” on white dwarf surfaces. Metals, in astronomy, means all the elements of the periodic table other than hydrogen and helium. But this is the first time they’ve found a literal scar on a star left by the cannibalization process.

The scarred white dwarf’s designation is WD 0816-310. It’s located 63 light-years from Earth. When it was younger, it was likely a star much like our sun (although perhaps more massive). But when it began to age, the star swelled up into a red giant star, just as our sun will do. Its outer layers must have engulfed any close-orbiting planets. When our sun becomes a red giant, its outer layers will engulf Earth and possibly Mars. Later, WD 0816-310 shrank down into a tiny burned-out corpse of its former self, called a white dwarf. Like most other known white dwarfs, WD 0816-310 is more massive than our sun, but only about the size of Earth.

And now we see that this white dwarf star has a metal scar (or “patch” as the scientific paper calls it).

These astronomers made their discovery using the Very Large Telescope (VLT) in Chile, which is operated by the European Southern Observatory (ESO).

The research team published their peer-reviewed results in The Astrophysical Journal Letters on February 26.

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Metal scar on white dwarf star

So astronomers had detected traces of metals on white dwarfs before, even from what they’ve believed were former Earth-sized planets. But this is the first actual scar they’ve found.

Additionally, they say that the white dwarf’s magnetic field must have been involved in creating the scar. They believe this to be the case because the scar is located at one of the white dwarf’s magnetic poles. Stefano Bagnulo, lead author and astronomer at Armagh Observatory and Planetarium in Northern Ireland, U.K., said:

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.

Co-author Jay Farihi at University College London, U.K., explained:

We have demonstrated that these metals originate from a planetary fragment as large as or possibly larger than Vesta, which is about 300 miles (500 km) across and the second-largest asteroid in the solar system.

White dwarf star: Bright white sphere with dark spot and looping lines around it. Many dark rock-like objects are nearby.
View larger. | This artist’s illustration shows the white dwarf star WD 0816-310, where astronomers have found a scar imprinted on its surface, the result of its having ingested planetary debris. Image via ESO/ L. Calçada.

How did the scar form?

Bagnulo and his colleagues used the FORS 2 instrument on the Very Large Telescope to find the scar. The astronomers refer to it as a “Swiss Army knife,” capable of various kinds of observations. Bagnulo said:

ESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs, and sensitively measure stellar magnetic fields.

The researchers found the scar by first noticing that the amount of metal detected varied as the white dwarf rotated. The metals were concentrated in one region instead of being spread out. Another clue came from how the concentration of metals was synchronized with the white dwarf’s magnetic field. The scar is on one of the white dwarf’s magnetic poles. The researchers said this means that the magnetic field ionized and “funneled” the metals from the former planet onto the magnetic poles of the white dwarf. Thus, the formation of the scar. The process is also similar to how auroras form on Earth and on Jupiter.

This is an artist’s animation of the white dwarf WD 0816-310 ingesting planetary fragments. Video via ESO/ YouTube.

Funneled instead of dispersed

Previous theories stated that such metals should be evenly distributed on the surface of a white dwarf. In fact, when astronomers have found metals on other white dwarfs, those metals have been scattered over the surface. But in the case of WD 0816-310, the magnetic field ionized the metals and guided them onto the magnetic poles. Co-author John Landstreet at Western University, Canada, said:

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.

Bottom line: Astronomers have discovered a unique metal scar on a cannibal star, a white dwarf or “dead” star. They believe it formed when the star consumed one of its planets

Source: Discovery of Magnetically Guided Metal Accretion onto a Polluted White Dwarf


Read more: Evidence for white dwarfs consuming Earth-like worlds

Read more: This white dwarf star has a giant, evaporating planet

March 5, 2024

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