Astronomers have discovered an amazing new type of star covered in burning ash helium

The artist’s impression of the rare fusion of the stars of two white dwarfs. Written by Nicole Raindle

A team of German astronomers led by Professor Klaus Werner of the University of Tübingen has discovered an amazing new type of star coated with the by-product of helium combustion. It is possible that stars could have formed as a result of a rare fusion of stars. Exciting results are published in Monthly reports of the Royal Astronomical Society.

While the surfaces of ordinary stars consist of hydrogen and helium, the surfaces of stars discovered by Werner and his colleagues are covered with carbon and oxygen, and the ashes of burning helium are an exotic composition for the star. The situation becomes more obscure as new stars have temperatures and radii that indicate they are still burning helium in their nuclei – a property commonly observed in more developed stars than those observed by Werner and his team in this study.

The second work of a group of astronomers from the University of La Plata and the Max Planck Institute for Astrophysics, published together with the work of Professor Werner and his team, offers a possible explanation for their formation. “We believe that the stars discovered by our German colleagues could have formed in a very rare form of stellar fusion of the two[{” attribute=””>white dwarf stars,” says Dr. Miller Bertolami of the Institute for Astrophysics of La Plata, lead author of the second paper. White dwarfs are the remnants of larger stars that have exhausted their nuclear fuel, and are typically very small and dense.

Stellar mergers are known to happen between white dwarfs in close binary systems due to the shrinking of the orbit caused by the emission of gravitational waves. “Usually, white dwarf mergers do not lead to the formation of stars enriched in carbon and oxygen,” explains Miller Bertolami, “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.”

Yet no current stellar evolutionary models can fully explain the newly discovered stars. The team needs 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.

“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,” explains Professor Werner.

Reference: ” Discovery of hot subdwarfs covered with helium-burning ash” by Klaus Werner, Nicole Reindl, Stephan Geier and Max Pritzkuleit, 12 February 2022, Monthly Notices of the Royal Astronomical Society: Letters.
DOI: 10.1093/mnrasl/slac005 Astronomers have discovered an amazing new type of star covered in burning ash helium

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