Study results suggest that when two stellar partners finally merge completely at the end of their lives, surprising results can occur: A special merger of white dwarfs could explain the formation of an unusual type of star, which astronomers are now reporting. While normal star surfaces are composed of hydrogen and helium, these stars are covered with carbon and oxygen – the “ash” from helium fusion processes. Presumably, this is precipitation material from one of the two partners, which “crumbled” during the fusion.
The astronomers led by Klaus Werner from the University of Tübingen have their sights set on the burnt-out remains of stars, or specimens that only emit small “signs of life”: white dwarfs and their precursors. These are the hot but faint remnants of sun-like stars near the end of their lives. To explain: The typical life cycle of a star like our sun begins with the nuclear fusion of hydrogen into helium. Later, a nuclear reaction begins inside the star, converting helium into carbon and oxygen. The star then expands into a red giant. It then evolves into a white dwarf: nuclear fusion processes cease and the rest of the star shrinks into a dense, smoldering remnant.
The end of stellar life in sight
In order to study the final phases of stellar evolution, Werner and his colleagues used the Large Binocular Telescope in Arizona to specifically search for dying stars. With the two large primary mirrors, each 8.4 meters in diameter, it was possible to capture the faint light that reaches us from these celestial bodies. In this way, the astronomers were able to analyze features of the light spectrum and thus draw conclusions about which elements are present in the stars. They came up with surprising results for two so-called hot sub-dwarfs: While the surfaces of stars normally consist of hydrogen and helium, these stars are covered with carbon and oxygen – i.e. the ash from helium nuclear fusion processes.
“Stars with the chemical surface composition of the discovered stars are usually expected to have completed helium fusion at the center and are close to the final stage of their development into white dwarfs,” explains Werner. The reason for the strange composition initially seemed possible that there was an explosive resumption of helium fusion, which then transported the burned ash – carbon and oxygen – from the interior to the surface. But as the astronomers explain, this seems unlikely with the types that have now been discovered – because other characteristics do not match: “They have too large radii and still carry out the helium fusion peacefully in their center,” says Werner. Their characteristics thus appear puzzling.
Result of a special fusion?
However, a possible explanation for the formation of these atypical stars is provided by the results of a team of astronomers led by Marcelo Miller Bertolami from the University of La Plata in Argentina. They were presented as a companion publication to the study by Werner and his colleagues in the same magazine. “We believe that the stars that our German colleagues discovered were formed by a very rare type of merger between two white dwarfs,” says Bertolami. Accordingly, the strange specimens were originally two stars that once orbited each other.
Mergers in binary systems are already known. A merger of white dwarfs can occur because the distance between their orbits is constantly decreasing due to the emission of gravitational waves, the scientists explain. “However, such mergers do not usually lead to the formation of a star enriched with carbon and oxygen,” says Bertolami. “However, we believe that in binary systems with very specific stellar masses, a white dwarf with a carbon-oxygen core can be ruptured by tidal forces. Its material is then dumped onto the surface of its white dwarf companion, leading to the formation of these exotic stars.
The two teams of scientists emphasize, however, that this is only an explanatory approach that needs to be further examined. This is because current models of stellar evolution cannot precisely describe the assumed process. The scientists are therefore now working on more refined models. They could not only help to better understand these types of stars, but also provide a deeper insight into the late evolution of binary star systems in general. “These new stars pose a major challenge to our understanding of stellar evolution,” concludes Werner.
Source: Royal Astronomical Society, University of Tübingen. Article: Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnrasl/slac005
Monthly Notices of the Royal Astronomical Society, doi: 10.1093/mnrasl/slab134