From the Heidelberg University Center for Astronomy (ZAH), Dr. Varsha Ramachandran and her colleagues discovered the first “fragmented” medium-mass star. The discovery marks a missing link in our picture of the evolution of stars into neutron star merging systems, which are crucial to our understanding of the origin of heavy elements such as silver and gold. Dr. Dr. Ramachandran at the ZAH Institute for Astronomical Research (ARI). He is a postdoc in Andreas Sander’s research group. These results have been published Astronomy and Astrophysics .

A team of researchers has discovered the first representative of a long-predicted but yet unconfirmed population of truncated intermediate-mass stars. “Deprived stars” are stars that have lost most of their outer layers, revealing their hot and dense helium-rich core as a result of nuclear fusion of hydrogen into helium. Most of these stars form in binary star systems, where a star’s strong gravity strips and deposits material from its companion.

Astrophysicists have long been familiar with low-mass stars known as sub-dwarfs, as well as their larger cousins ​​known as Wolf-Rayet stars. But so far, they haven’t found any of the so-called “intermediate-mass fission stars,” which raises the question of whether our basic theoretical picture needs a serious overhaul.

Studying hot and bright stars with the High Resolution Spectroscopy instruments of the European Southern Observatory’s Very Large Telescope VLT in Chile, Dr. Ramachandran and colleagues discovered suspicious features in the spectrum of a hot-mass star that was previously classified as a single. article. A detailed examination of the spectrum showed that the object was not a single star, but rather a binary system consisting of a medium-mass star and a fast-spinning companion called the Be star, which rotates due to the increase. in mass. from a naked star ancestor.

The system is located in a nearby dwarf galaxy called the Small Magellanic Cloud (SMC). The stars in this galaxy contain less of the heavy elements that astrophysicists call simply “metals” than the massive stars in our Milky Way. Thus, the metal-poor mass stars in the SMC serve as a window into the past of our own galaxy and the chemical evolution of the Universe.

Dr. Ramachandran studied in India and then moved to Potsdam, Germany to complete his PhD. He has been working at ZAH/ARI since September 2021. “With our discovery, we show that a long-lost population of such stars does indeed exist. Ramachandran added that such stars are able to retain a small but sufficient amount of hydrogen above their helium cores instead of completely losing their outer layers, causing them to appear much larger and colder than they are.

“That’s why we call them ‘partially absent stars,'” he adds. Dr. Andreas Sander now states that hydrogen shrouds are a form of camouflage. “Partly exposed stars look a lot like normal, unexposed hot stars, so they’re essentially hiding in sight. Only high-resolution data combined with careful spectral analysis and detailed computer models can reveal their true nature.”

No wonder they avoided detection for so long. “The particular advantage of this star was its mass: It may appear several times larger than our Sun, but it is extremely light for a blue supergiant,” explains the lead of the research team.

An independent scientist at the European Southern Observatory (ESO) and co-author of the corresponding scientific paper, Dr. Jakub Klentzki explains that the newly discovered system serves as a critical link in the evolutionary chain that connects several different “types” of exotic objects. . . “Our stellar evolution models predict that after about a million years the naked star will explode as a so-called truncated supernova, leaving behind a neutron star remnant,” says Dr Klenkey.

Dr. The discovery by Ramachandran and colleagues marks the first such bare star ever found in a metal-poor galaxy. If the binary system survives the supernova explosion, the roles of the two stars are reversed: the Be star companion then gives mass to the neutron star collector, becoming the X-ray binary Be star.

Such fascinating systems are believed to be precursors to double neutron star mergers, perhaps the largest cosmic spectacle ever observed, and the origin of chemical elements such as silver or gold. Understanding the ways of their formation is one of the main tasks of modern astrophysics, and observing intermediate stages of evolution is crucial to achieving this.

Dr. “Our discovery adds an important piece to the puzzle by providing the first direct constraints on how mass transfer evolves in such massive stellar systems,” Ramachandran said. Source