First intermediate-mass stripped star discovered

Dr. Varsha Ramachandran and her colleagues from the Center for Astronomy at Heidelberg University (ZAH) have made a significant discovery in the field of stellar evolution. They have identified the first “stripped” star of intermediate mass, filling a crucial gap in our understanding of stellar evolution and the formation of neutron star mergers, which play a vital role in the creation of heavy elements like gold and silver. Dr. Ramachandran, a postdoctoral researcher working under the guidance of Dr. Andreas Sander at ZAH’s Astronomisches Rechen-Institut (ARI), recently published these findings in the journal Astronomy & Astrophysics.

The team’s breakthrough came with the identification of the first known representative from the predicted yet unconfirmed population of intermediate-mass stripped stars. These stars, referred to as “stripped” due to the loss of their outer layers, exhibit a hot and dense core rich in helium resulting from the fusion of hydrogen. Most stripped stars form in binary star systems, where one star’s gravitational pull causes it to strip and accumulate matter from its companion.

While astrophysicists have long been aware of low-mass stripped stars, known as subdwarfs, as well as their high-mass counterparts called Wolf-Rayet stars, the existence of intermediate-mass stripped stars remained elusive. This discovery raises questions about the need for significant revisions to our current theoretical understanding.

To make this breakthrough, Dr. Ramachandran and her team employed high-resolution spectroscopy devices on the Very Large Telescope (VLT) at the European Southern Observatory in Chile. Through this survey, they detected peculiar features in the spectrum of a hot, massive star previously classified as a single object. A thorough investigation of the spectrum revealed that the object was, in fact, a binary system comprising an intermediate-mass stripped star and a fast-rotating companion known as a Be star. The Be star had acquired its rapid rotation by accreting mass from the progenitor of the stripped star.

This binary system was found in the Small Magellanic Cloud (SMC), a nearby dwarf galaxy. Stars in the SMC have lower metal abundances compared to those in our Milky Way galaxy. Consequently, the metal-poor massive stars in the SMC provide valuable insights into the past of our own galaxy and the chemical evolution of the universe.

Schematic illustration of the evolution of the newly discovered massive binary system towards a double neutron star merger event. A stripped-envelope supernova happens in between followed by a Be X-ray binary. Credit: Varsha Ramachandran, ZAH/ARI

Dr. Varsha Ramachandran completed her undergraduate studies in India before pursuing her Ph.D. in Potsdam, Germany. Since September 2021, she has been conducting research at ZAH/ARI. According to Dr. Ramachandran, their recent discovery demonstrates the existence of the long-awaited population of stripped stars, but it also suggests that these stars may differ significantly from initial expectations. Instead of being completely devoid of their outer layers, the stripped stars retain a small but significant amount of hydrogen above their helium cores, resulting in their larger and cooler appearance.

As a result, Dr. Ramachandran refers to them as “partially stripped stars.” Dr. Andreas Sander explains that the remaining hydrogen layer acts as a disguise, making these stars appear similar to normal, non-stripped hot stars. To unveil their true nature, it requires high-resolution data, meticulous spectral analysis, and detailed computer models.

The discovery of these partially stripped stars has been challenging due to their deceptive appearance. Dr. Sander highlights that while the star in question had a mass several times that of the Sun, it appeared extraordinarily light for a blue supergiant.

Dr. Jakub Klencki, an independent research fellow at the European Southern Observatory (ESO) and co-author of the study, emphasizes that this newfound system serves as a crucial link in understanding the evolutionary sequence of various exotic objects. According to their stellar evolution models, the stripped star will eventually undergo a stripped-envelope supernova in approximately one million years, leaving behind a neutron star remnant.

Notably, this stripped star is the first of its kind discovered in a metal-poor galaxy. If the binary system survives the supernova explosion, the roles of the two stars will reverse, with the Be-star companion transferring mass to the accreting neutron star, resulting in a Be X-ray binary.

These intriguing systems are believed to be progenitors of double neutron star merger events, which are among the most awe-inspiring cosmic phenomena observed thus far and the source of precious elements like silver and gold. Understanding their formation is a significant challenge in modern astrophysics, and observations of intermediate evolutionary stages play a crucial role.

Dr. Ramachandran concludes that their discovery provides valuable insights into the mass transfer evolution within massive star systems, serving as a major piece of the puzzle in understanding the formation process.

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