Their stellar partner has stripped them of their hydrogen shell: astronomers have confirmed the existence of a special category of “denuded” stars in binary systems. The scientists explain that the identification of 25 such candidates supports theoretical assumptions about the cause of a cosmic phenomenon: The hydrogen-free stars are apparently the hot helium stars of medium mass, which are assumed to be the precursors of hydrogen-poor ones Form supernovae.
With the naked eye we only see a single point in the sky - but often there are actually two: astronomical research has now clearly shown that there are many binary star systems in the cosmos. Two stars orbit each other in a kind of cosmic dance and have a significant influence on their further development. This also applies to the final phase of their existence. When stars of a certain mass begin to burn out, they expand into a red giant, around which a hydrogen-rich shell forms. In the case of individual stars, a supernova explosion ultimately occurs, in the bright light of which the spectral signature of hydrogen can also be seen.
So far only theory
But in binary systems, according to theoretical assumptions, a special process can occur: If one of the stellar partners expands into a red giant, the other can use its attraction to remove the hydrogen-rich envelope. In the end, only a helium core remains, which contracts and develops temperatures more than ten times higher than the sun. With the appropriate mass, this helium star can also explode in a supernova - which then lacks the signature of hydrogen. Such hydrogen-poor supernovae even form around a third of all stellar explosions, according to observational data.
This also seems to be consistent with the fact that, according to model simulations, about every third medium-mass star in binary star systems loses its hydrogen shell. In other words: The possible precursors of hydrogen-poor supernovae should actually be found frequently. But until now this star category remained hidden. Only in one case has there been at least corresponding evidence so far. “If these stars had turned out to be rare, it would have called into question an important theoretical framework. “But we have now been able to confirm that these stars actually exist,” says lead author Maria Drout from the University of Toronto.
On the trail of hot helium stars
Together with co-first author Ylva Götberg from the Institute of Science and Technology Austria in Klosterneuburg and an international team, Drout tracked down the stars they were looking for based on their special radiation: They developed a method to examine the ultraviolet part of the spectrum Extremely hot celestial bodies such as helium stars emit most of their light. They analyzed data from NASA's Swift Ultra-Violet/Optical Telescope, which focused on two of the closest galaxies to Earth: the Large and Small Magellanic Clouds. The search for stars with unusual UV emissions ultimately led to 25 candidates. These were then subjected to a more detailed spectroscopic study using the Magellan telescopes at the Las Campanas Observatory.
The scientists were finally able to confirm that the stars they had identified are actually very hot, small and low in hydrogen and are located in binary systems. So they meet all the criteria of the model predictions. “Strong spectral signatures of ionized helium gave us important clues: first, they confirm that the outermost layers of stars are dominated by helium, and second, that their surfaces are very hot. “This is exactly what is expected for stars that have an exposed, compact, helium-rich core after mass transfer,” says Götberg.
The scientists were now able to find the previously hidden medium-mass helium stars that were predicted to be the precursors of the hydrogen-poor supernovae. “There are probably still a lot of them to be discovered,” says Götberg. That's why, in addition to examining the stars discovered so far in more detail, the team is now also planning to expand their search: They want to use additional telescopes to look for more candidates in this interesting star category in more galaxies and, above all, in our Milky Way.
Source: University of Toronto, Institute of Science and Technology Austria, specialist article: Science, doi: 10.1126/science.ade4970