Professor Klaus Werner of the University of Tübingen led a team of German astronomers that found a weird new form of star coated with helium-burning by-product. A rare stellar merger event could have formed the stars. Monthly Notices of the Royal Astronomical Society has published the fascinating findings.
Unlike normal stars, which have hydrogen and helium on their surfaces, the stars discovered by Werner and his colleagues have carbon and oxygen on their surfaces, with helium ashes on top — an unusual composition for a star. The issue gets even more perplexing when the new stars have temperatures and radii that suggest they are still burning helium in their cores, a trait generally found in more developed stars than the ones studied by Werner and his colleagues.
A second paper by astronomers from the University of La Plata and the Max Planck Institute for Astrophysics, which was published alongside Professor Werner’s and his team’s work, suggests a possible explanation for their formation. “We think the stars identified by our German colleagues occurred in a very uncommon kind of stellar merger event between two white dwarf stars,” explains Dr. Miller Bertolami of the Institute for Astrophysics of La Plata, the second paper’s main author. White dwarfs are the remains of bigger stars that have run out of nuclear fuel, and they are generally tiny and dense.
Because of the shrinking of the orbit caused by gravitational wave emission, stellar mergers have been observed between white dwarfs in close binary systems. “We think that, for binary systems produced with very particular masses, a carbon- and oxygen-rich white dwarf can be disturbed and end up on top of a helium-rich one, leading to the birth of these stars,” says Miller Bertolami.
However, none of the existing stellar evolutionary models can fully explain the newly discovered stars. In order to determine if these mergers are feasible, the team will need to enhance its models. These models may not only aid in the team’s understanding of these stars, but they may also shed light on the late evolution of binary systems and how their stars exchange mass as they progress. The origin of the helium-covered stars will be a point of contention until astronomers develop more refined models for binary star evolution.
“Normally, stars with these surface compositions would have burned all of the helium in their cores and would be on their way to becoming white dwarfs. These newly discovered stars pose a serious threat to our understanding of stellar evolution “Professor Werner elucidates the situation.