While normal stars have surfaces composed of hydrogen and helium, the stars discovered by Werner and his colleagues have their surfaces covered with carbon and oxygen, the ashes of helium burning — an exotic composition for a star.
The situation becomes more puzzling as the new stars have temperatures and radii that indicate they are still burning helium in their cores — a property typically seen in more evolved stars than those observed by Werner and his team in this study.
What makes these two stars unusual is the high levels of carbon and oxygen at the surface of their atmospheres. Those elements are left behind as a star burns its helium, but that process takes place in the star’s core and wraps up long before carbon and oxygen start to dominate the star’s atmosphere. Weirdly, these stars seem to still be running through helium despite their odd surfaces.
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“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,” Klaus Werner, an astronomer at the University of Tübingen in Germany and lead author of one new study of these strange stars, said in a statement from the Royal Astronomical Society (RAS), which published the new research in its journal. “These new stars are a severe challenge to our understanding of stellar evolution.”
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“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,” says Dr Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper.
Specifically, the scientists looked at two stars dubbed PG1654+322 and PG1528+025. Except for the high levels of carbon and oxygen in their atmospheres, these two stars look like other small, hot stars on their way to becoming white dwarfs, the dense “stellar corpses” left behind when small and medium stars run out of fuel to burn.
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Yet no current stellar evolutionary models can fully explain the newly discovered stars. The team need refined models in order to assess whether these mergers can actually happen. These models could not only help the team to better understand these stars, but could also provide a deeper insight into the late evolution of binary systems and how their stars exchange mass as they evolve.
Until astronomers develop more refined models for the evolution of binary stars, the origin of the helium covered stars will be up for debate.
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And yet, because of the size and temperatures of these two stars, astronomers believe these objects are still burning helium. But usually, stars sport the light elements hydrogen and helium at their surfaces, not the much heavier carbon and oxygen.
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According to the team, carbon and oxygen are normal in old stars that are fusing helium, but only in their cores. So it is extremely unusual to see them in large quantities at their surface.
“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,” Marcelo Miller Bertolami, an astronomer at the Institute for Astrophysics of La Plata in Argentina and lead author of the second paper, said in the RAS statement.
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The typical life cycle of a star like our Sun begins with the nuclear fusion of hydrogen into helium. Then, deep inside the star, a nuclear reaction begins that converts helium into carbon and oxygen. The star ‘dies’ in the course of millions of years and shrinks to a ‘white dwarf’ – a small, very dense star that is typically the size of a planet.
“Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” Miller Bertolami said, “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.”
Stars covered with carbon and oxygen instead of hydrogen is thought to be due to an explosive resumption of helium fusion, which then brings the burning ash – carbon and oxygen – to the surface.