German astronomers have discovered a new type of star, and exactly how these weird white dwarfs came to be remains a mystery. The stars are covered in a layer of “ash” that’s usually produced by burning helium, indicating they may have formed through collisions between other stars.
When stars in a certain mass range run out of fuel and explode, they leave behind a dense core that can no longer undergo fusion. This remnant, called a white dwarf, slowly cools to the background temperature of the universe over the next few trillion years.
But now, astronomers have discovered two white dwarfs that don’t quite fit the usual description. White dwarfs have atmospheres dominated by hydrogen or helium, but these new ones have surprisingly high amounts of carbon and oxygen in their atmospheres – rather than the usual trace amounts if anything, the team detected concentrations of both elements that were as high as 20 percent.
Intriguingly, carbon and oxygen are the “ashes” produced when stars burn helium, something that white dwarfs are supposed to have long finished doing. Even more puzzling is that these new stars are hotter and wider than most white dwarfs, indicating that they may still be burning helium in their cores.
“Normally we expect stars with these surface compositions to have already finished burning helium in their cores, and to be on their way to becoming white dwarfs,” said Professor Klaus Werner, lead researcher on the team. “These new stars are a severe challenge to our understanding of stellar evolution.”
The researchers do have a hypothesis for how the strange new stars were born. Pairs of white dwarfs in close binary systems can sometimes pull each other in closer together until they collide, forming a new object. If the compositions of each original white dwarf were just right, the end result could be the new stars seen here.
“Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” said Dr. Miller Bertolami, lead author of the study. “But we believe that, for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf might be disrupted and end up on top of a helium-rich one, leading to the formation of these stars.”
Even so, the model doesn’t explain all the observed features of these new stars. The team says that more work will need to be done to investigate their origin.
These aren't the only perplexing white dwarfs out there. Other recent studies have found white dwarfs with ring systems, white dwarf pulsars, and a particularly bizarre one that's somehow burning bright in infrared but not visible light.
The research was published in two studies, both appearing in the journal Monthly Notices of the Royal Astronomical Society.
Source: Royal Astronomical Society