WASHINGTON. — A slowly cooling stellar ember called a white dwarf with a scar on its face is providing new insight into the behavior 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, 5.9 trillion miles (9.5 trillion km). Like all white dwarfs, it is incredibly dense, packing about 70% 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 white dwarfs. 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 funneling 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.
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 330 miles (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. — Reuters