The light giant reveals a missing evolutionary stage

Dr. Varsha Ramachandran of the Center for Astronomy at the University of Heidelberg (ZAH) and colleagues have revealed the first “massless” medium-mass star. The discovery represents a missing link in our picture of stellar evolution toward merging neutron-star systems, which is essential to our understanding of the origin of heavy elements, such as silver and gold. Dr. Ramachandran is a postdoctoral researcher in the research group of Dr. Andreas Sander, based at ZAH’s Reschen Astronomy Institute (ARI). These results are now published in Astronomy and astrophysics.

The team of researchers has discovered the first representative of long-predicted, but not yet confirmed, intermediate-mass stars. “Truncate stars” are stars that have lost most of their outer layers, exposing their hot, dense, helium-rich cores, which result from the nuclear fusion of hydrogen with helium. Most of these naked stars form in binary star systems where the strong gravity of one star flakes off and material from its companion accumulates.

For a long time, astrophysicists have known the abstract, low-mass stars, known as sub-dwarfs, as well as their massive cousins, known as Wolf-Rayet stars. But so far, they’ve never been able to find any of these so-called “naked medium-mass stars,” which raises questions about whether our basic theoretical picture needs a major revision.

By scanning hot, bright stars with the high-resolution spectroscopy instruments of the European Southern Observatory’s VLT, Very Large Telescope in Chile, Dr. Ramachandran and her colleagues detected suspicious signals in the spectrum of a once massive hot star. Classified as a single object. A detailed investigation of the spectrum revealed that the object is not a single star but is actually a binary system, consisting of a medium-mass star and a rapidly rotating attachment, a so-called be star that was wrapped by the accumulating mass of the star’s naked progenitor.

The system is located in a neighboring dwarf galaxy called the Small Magellanic Cloud (SMC). The stars in this galaxy have a lower abundance of heavier elements, which astrophysicists simply call “metals,” than the massive stars in our Milky Way. Thus, the massive, metal-poor stars in the SMC serve as a window into our galaxy’s past and the chemical evolution of the universe.

Dr. Ramachandran completed her undergraduate studies in India, before moving to Potsdam, Germany to obtain her Ph.D. She has been working since September 2021 at ZAH / ARI. “With our discovery, we prove that long-lost clusters of these stars do indeed exist. But our findings also suggest that they may look very different from what we expected,” explains Dr. Ramachandran, who adds that instead of completely losing their outer layers, they may retain Such stars have a small but sufficient amount of hydrogen over their helium cores, which makes them appear much larger and cooler than they really are.

“So we call them ‘partially naked stars,’” she adds. Dr. Andreas Sander points out that the remaining mantle of hydrogen is a form of disguise. “Partially naked stars look very similar to normal, non-naked hot stars, and so are essentially hiding in plain sight. Only high-resolution data combined with careful spectral analysis and detailed computer models can reveal its true nature.”

Not surprisingly, they evaded detection for so long. “The special gift of this star was its mass: it may appear to be a few times more massive than our Sun, but this is unusual light for its giant blue appearance,” explains the research group leader.

Dr. Jacob Klinke, an independent research fellow at the European Southern Observatory (ESO) and co-author of the research paper in question, explains that the newly discovered system serves as a critical link in the evolutionary chain that connects several different “species” of alien organisms. “Our models of stellar evolution predict that about a million years from now, the naked star will explode as a presumably enveloped supernova, leaving behind a neutron star remnant,” says Dr. Klinke.

The discovery made by Dr. Ramachandran and her colleagues marks the first naked star yet to be found in a mineral-poor galaxy. If the binary survives the supernova explosion, the roles of the two stars will reverse: The Be-star companion will then donate mass to the neutron star complex, becoming a so-called Be-ray binary.

Such fantastic systems are considered to be the progenitors of double neutron star merger events, perhaps the greatest cosmic scenes observed to date and the origin of chemical elements such as silver or gold. Understanding their path of formation is one of the major challenges of modern astrophysics, and observations of intermediate evolutionary stages are essential to achieving this.

“Our discovery adds a major piece to the puzzle, yielding the first direct constraints on how the evolution of mass transfer proceeds in such massive stellar systems,” concludes Dr. Ramachandran.