Using the Large Binocular Telescope, a team of German astronomers has discovered two unusual hot subdwarf stars named PG 1654+322 and PG 1528+025 . They are so unusual because, while normal stars have surfaces made of hydrogen and helium, they appear to be covered with ashes of carbon and oxygen, the byproduct of helium nuclear fusion.
The experts, who publish their research in the Monthly Notices of the Royal Astronomical Society: Letters , speak of “staggeringly high abundances” of both carbon and oxygen, with around 20% of the surface composition of both stars. The helium nucleus continues to fuse within it, an unprecedented discovery.
These unusual stars are within our galaxy but are between 10,000 and 25,000 light-years away from Earth.
The researchers believe that this new type of star was formed by the merger of two white dwarfs , the hot, dense remains of dead stars.
“Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” explains Miller Bertolami, co-author of the paper. “But we think that for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf could break up and end up on top of a helium-rich one, leading to the formation of these stars.”
How to explain this finding?
Astronomers need more refined models to assess whether these mergers can actually occur, which could shed light on the origin of helium-coated stars . There is no stellar evolutionary model that can explain these newly discovered stars.
It is known that stellar mergers between white dwarfs in close binary systems can take place due to the reduction of the orbit caused by the emission of gravitational waves, but this phenomenon does not generate a star enriched in carbon and oxygen (it is normal that there are carbon and oxygen in old stars by fusing helium, but only in the core, not on their surface). So, for now, the debate remains open.
“These new stars are a severe challenge to our understanding of stellar evolution,” concludes Klaus Werner, leader of the work.
Referencia: Klaus Werner et al, Discovery of hot subdwarfs covered with helium-burning ash, Monthly Notices of the Royal Astronomical Society: Letters (2022). DOI: 10.1093/mnrasl/slac005
