Astronomers observe scar on white dwarf 'cannibal' star

A slowly cooling stellar ember called a white dwarf with a scar on its face is providing new insight into the behaviour of certain “cannibal” stars at the end of their life cycle.

Using the European Southern Observatory’s Chile-based Very Large Telescope, researchers studied a white dwarf located about 63 light years from Earth. A light year is the distance light travels in a year, 9.5 trillion km. Like all white dwarfs, it is incredibly dense, packing about 70 per cent of the sun’s mass into an Earth-sized object.

Stars with up to eight times the mass of our sun appear destined to end up as a white dwarf. They eventually burn up all the hydrogen they use as fuel. Gravity then causes them to collapse and blow off their outer layers in a “red giant” stage, eventually leaving a compact core – the white dwarf.

Astronomers have established that white dwarfs ingest fragments of planets and moons as well as asteroids. In the new study, the researchers detected for the first time a telltale sign of this process – a scar on the white dwarf’s surface made up of the metal elements of a planetary fragment or asteroid that it gobbled – accreted, in scientific terms – after being funnelling in by the star’s magnetic field.

The researchers were surprised by the finding, having suspected that the debris would have blended with the rest of the material on the white dwarf’s surface.

“We did not think that the magnetic field could prevent the accreted material from mixing on the surface of the star. When you pour sugar in a glass of water, all water becomes sweet,” said astronomer Stefano Bagnulo of Armagh Observatory and Planetarium in Northern Ireland, lead author of the study published on Monday in the Astrophysical Journal Letters.

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It was unclear what type of body had left the scar, which included iron, nickel, titanium, chromium, magnesium and other elements.

“This particular ‘planet snack’ was at least as massive as Vesta, the second-largest asteroid in our solar system,” University College London astronomer and study co-author Jay Farihi said.

Vesta is a rocky object in our solar system’s main asteroid belt with a diameter of about 530 km.

“Planetary systems are born together with their star, all condensing from a cloud of dust and gas. We often call the star the ‘parent,’ so this is a bit like a mother eating her children,” Farihi added.

This white dwarf started its life as a star about twice the sun’s mass, living a lifespan of perhaps 1.2 billion years before entering its death throes.

Many white dwarfs have a debris disk orbiting them – the remnants of a planetary system. This material gradually falls onto the star’s surface.

“We say the atmosphere of these stars is ‘polluted’ by metal elements,” Bagnulo said.

About 20 per cent of white dwarfs possess a strong magnetic field. Some white dwarfs, like this one, have both traits – they have an atmosphere polluted by metal elements and are permeated by a magnetic field.

“The key discovery is that we have seen that the magnetic field plays a central role in the way the disk debris falls at the surface of the star. Not only is the material funneled by the magnetic field, but it is also stuck at the magnetic poles, without being mixed at the surface of the star,” Bagnulo said.

This bleak vision of a star’s end may be more than merely academic for us Earthlings. It could be a vision of the future of our own solar system – though many, many years away. The sun is roughly 4.5 billion years old.

“Our sun will become a white dwarf in 5 billion years,” Farihi said, “and will likely become polluted by our planetary system.” REUTERS