Astronomers have discovered a new type of star that is utterly puzzling. The stars are covered in oxygen and carbon instead of having a surface made of hydrogen and helium. Astronomers think that they may be the end product of a peculiar and rare stellar collision.
To understand why this is peculiar, we need to consider how stars make elements. Stars for most of their lives are fusing hydrogen atoms in their core into helium. If they are massive enough, when the hydrogen runs out, stars transform into red giants and start fusing helium into carbon and oxygen. If they are not massive enough they leave behind a helium white dwarf, which has just traces of carbon, oxygen, and nitrogen, elements that are the ashes of helium burning.
The same happens to red giants, if they are not massive enough to go on to the next stage of fusion, they leave behind a carbon-oxygen-rich white dwarf. But the stars reported in a new paper in the Monthly Notices of the Royal Astronomical Society Letters have an exterior rich in oxygen and carbon, about 20 percent each of the surface composition. Underneath this peculiar surface, the two stars have a lot of helium.
“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. These new stars are a severe challenge to our understanding of stellar evolution,” lead author Professor Klaus Werner, from the University of Tübingen, said in a statement.
So what gives? A possible solution was provided by astronomers from the University of La Plata and the Max Planck Institute for Astrophysics. In a second paper, also published in the Monthly Notices of the Royal Astronomical Society Letters, another team suggests that a merger between white dwarfs could create the unusual stars discovered.
“We believe the stars discovered by our German colleagues might have formed in a very rare kind of stellar merger event between two white dwarf stars,” said Dr Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper. “Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen 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.”
Evolutionary models can’t explain this mechanism or the existence of these stars, so more investigating and examples, beyond stars PG1654+322 and PG1528+025 discussed in this research, is now needed.