Odd white dwarf found with mostly oxygen atmosphere

White dwarfs — the exposed cores of dead stars — are the last place astronomers expected to find an oxygen atmosphere. Yet that’s exactly what recently turned up, providing researchers a rare peek inside the core of a massive star and raising questions about how such an oddball could have formed.

Most stars die by gently casting the bulk of their gas into space, leaving behind a dense, hot core. Heavy elements such as carbon and oxygen sink to the core’s center while hydrogen and helium float to the surface. But a newly discovered white dwarf, about 1,200 light-years away in the constellation Draco, has no hydrogen or helium at its surface. Its atmosphere is instead dominated by oxygen, researchers report in the April 1 Science.
“We only found one, so it is a rare event,” says study coauthor Kepler de Souza Oliveira Filho, an astronomer at the Federal University of Rio Grande do Sul in Porto Alegre, Brazil. But, he says, “every theory must be able to explain all events, even the rare ones.”

Hydrogen and helium blanket most white dwarfs, hiding what lies beneath. Here, astronomers have “a window into the core of a star that we didn’t have before,” says Patrick Dufour, an astrophysicist at the University of Montreal.

While oxygen dominates this white dwarf’s atmosphere, neon and magnesium come in second and third — a clue that the original star was much bigger than our sun. Big stars can crank up their core temperatures high enough to fuse progressively heavier elements. A star between about six and 10 times as massive as the sun ends up with a core made of mostly oxygen, neon and magnesium — precisely what Filho and colleagues found. But there’s a problem: Such a white dwarf should be a bit heavier than our sun, and this newly discovered misfit appears to have about half as much mass.

A nearby stellar companion could have siphoned gas off the dying star, starving the white dwarf of mass, the researchers suggest. Thermonuclear excavation during the star’s end game could also lead to an underweight white dwarf. If enough hydrogen piled up on the core, it might have triggered a runaway nuclear explosion that shaved off the white dwarf’s outer layers.

While plausible, it’s hard to see how that could remove half of the white dwarf’s mass, Dufour says. “That’s very strange,” he says. “It could work, but I doubt it would leave a low-mass white dwarf.”
In 2007, Dufour and colleagues reported a similar strange sighting: several white dwarfs whose atmospheres were loaded with carbon instead of hydrogen and helium. Those also appeared to be missing some mass, he says, though the problem was found to lie not with the stars but with the mass estimates. The white dwarfs are heavier than initially thought, and Dufour now suspects that each one arose from a collision between two white dwarfs.

It’s too early to draw strong conclusions from a single oxygen-laden white dwarf. “There are lots of open questions before we can say that this changes our view of white dwarf evolution,” Dufour says. “This white dwarf might only be a freak…. Although often in science, it’s the exception that makes you understand a great deal later on.”