Astronomers have discovered a special type of small but super-hot stars, externally coated in carbon and oxygen, left over from the burning of helium. Usually, stars with this surface composition are considered to be completely burnt out and are on the way to becoming white dwarfs, but in this case, helium is still present in their cores and nuclear reactions are taking place. This new discovery could change scientists’ understanding of the possible paths of stellar evolution.
A group of German astronomers, led by Professor Klaus Werner of the University of Tübingen, have discovered a strange new type of star covered in the by-products of helium combustion. Perhaps these stars were formed as a result of a rare stellar merger. The research results are published in the journal Monthly Notices of the Royal Astronomical Society.
While the outer shells of ordinary stars are made up of hydrogen and helium, the stars discovered by Werner and his colleagues at the Institute for Physics and Astronomy at the University of Potsdam are covered in carbon and oxygen – a kind of ash from burned helium, from which carbon and carbon were synthesized as a result of thermonuclear reactions. oxygen. But this situation looks even more puzzling, as the temperatures and radii of new stars indicate that they are still burning helium in their cores.
The two superhot stars, designated PG1654+322 and PG1528+025, were discovered over 10,000 light-years from Earth using data from the Large Binocular Telescope (LBT) in Arizona and a survey by China’s LAMOST spectroscope. The surface of these dense stars is an order of magnitude hotter than that of the Sun.
Published together with the work of Professor Werner and his colleagues, an article by a group of astronomers from the Argentine National University of La Plata and the German Institute for Astrophysics of the Max Planck Society offers a possible explanation for the formation of such stars.
“We believe that the stars discovered by our German colleagues could have been formed as a result of very rare stellar mergers of pairs of white dwarfs,” says Dr. Miller Bertolami of the La Plata Institute of Astrophysics, lead author of the second paper. “In our paper, we argue that, under certain conditions, a carbon-oxygen white dwarf can be destroyed and accreted by a companion, forming objects like those discovered by Werner et al.”
White dwarfs are the remnants of larger stars that have exhausted their nuclear fuel; as a rule, they are very compact and dense formations that glow only due to the energy accumulated earlier. After a few billion years, the Sun will also be destined to become a white dwarf.
In order to become a white dwarf in the process of evolution, and not turn into a neutron star, a star in its mass should not exceed about ten times the mass of the Sun, and there are more than 97% of such stars in the Galaxy. When such a low-mass main-sequence star runs out of hydrogen and stops producing helium, it expands to become a red giant. The glow of a red giant is provided by thermonuclear reactions of the conversion of helium into carbon and oxygen. Then the star sheds its outer shells, the so-called planetary nebula is formed, and the former core turns into a white dwarf, consisting mainly of carbon and oxygen. If the initial mass of the star is not enough, fusion reactions can stop at helium, leading to the formation of helium white dwarfs. The average density of matter in white dwarfs is almost a million times that of main sequence stars.
Like other mergers of stars and quasi-stellar objects, mergers of white dwarfs occur in close binary systems due to the gradual approach of the two components caused by the loss of energy due to the emission of gravitational waves. “Normally, white dwarf mergers don’t result in the formation of stars rich in carbon and oxygen,” explains Miller Bertolami, “but we believe that for some binary systems with a very specific combination of masses, a white dwarf rich in carbon and oxygen could break up and cover a helium-rich white dwarf that has not yet been completely burnt out, which would lead to the formation of such unusual stars.
The hard-to-explain feature here is a special type of “accretion”: instead of two stars simply mixing all the material and becoming one single star, in this case the interaction should occur according to a different scenario – it’s like putting a glove on your hand – one the star seems to be pulled over another.
“We think that in binary systems with very specific stellar masses, a white dwarf with a carbon-oxygen core could be similarly torn apart by tidal forces. Its material is then dumped onto the surface of its white dwarf companion, which leads to the formation of these exotic stars,” says Dr. Nicole Reindl, an astronomer at the Institute for Astronomy and Astrophysics at the University of Potsdam, co-author of the first paper.
However, no modern models of stellar evolutions cannot fully explain the recent discovery. Astronomers will have to refine their stellar models to assess whether such mergers can actually occur. These refined models should not only help better understand the origins of these particular stars, but also provide deeper insight into the late evolution of binary systems and how their stars exchange mass as they evolve.
“Normally, we would expect stars with this surface composition to have finished burning helium in their cores and are on their way to becoming white dwarfs. These new stars pose a serious challenge to our understanding of stellar evolution,” says Professor Werner.
Until astronomers get these more accurate models of stellar evolution, the origin of stars covered in “helium ash” will remain a matter of debate.
