Researchers at the University of Toronto have identified a group of massive stars within binary systems that have undergone the stripping of their hydrogen envelopes by their companions. The discoveries provide insights into hot helium star, considered the progenitors of hydrogen-poor core-collapse supernovae and neutron star mergers.

For more than ten years, scientists have proposed that around one-third of massive stars undergo hydrogen envelope stripping in binary systems. However, until this recent revelation, only a single potential candidate had been recognized.

Binary-stripped stars refer to massive stars within binary systems that have undergone a process known as envelope stripping. In a binary system, two stars orbit around a common center of mass, influenced by each other’s gravitational forces. When one of these star is massive and evolves, it can transfer material, including its outer hydrogen envelope, to its companion star.

The stripping of the hydrogen envelope is a crucial aspect of this phenomenon. The outer envelope of a star, primarily composed of hydrogen gas, plays a significant role in the star’s evolution. When a massive star transfers this outer envelope to its binary companion, it exposes its core, which is typically rich in helium.

The discovery of binary-stripped stars is noteworthy because it allows astronomers to study hot helium stars. These stars are believed to be associated with specific astrophysical events, such as hydrogen-poor core-collapse supernovae and neutron star mergers. By identifying and studying these stripped stars within binary systems, astronomers can gain insights into the mechanisms driving stellar evolution and the processes leading to significant cosmic events.

Previously invoked to account for why a third of core-collapse supernovae exhibit significantly lower hydrogen content than typical Red Supergiant star explosions, binary-stripped stars are now proposed by Drout and her colleagues to eventually undergo hydrogen-poor supernova explosions. These star systems are also considered crucial for the formation of neutron star mergers, such as those detected by the LIGO experiment on Earth, which captures gravitational waves.

The researchers posit that some objects in their current sample are likely stripped stars with companions like a neutron star or black hole. These objects are believed to be in the stage just before evolving into double neutron star systems or systems involving a neutron star and a black hole, with the potential for eventual mergers.

As a star undergoes evolution, expanding into a red giant, the gravitational pull from a companion star can strip away the outer hydrogen layers of one, exposing a highly heated helium core. This process unfolds over tens of thousands, or even hundreds of thousands, of years.

Detecting stripped stars poses challenges because much of the emitted light falls outside the visible spectrum and may be obscured by cosmic dust or overshadowed by their companion star.

Initiating their exploration in 2016, Drout and her collaborators embarked on a quest. Drawing on her expertise in hydrogen-poor supernovae from her doctoral research, Drout, during a NASA Hubble Postdoctoral Fellowship at the Observatories of the Carnegie Institution for Science, aimed to locate the stripped stars believed to be central to such supernovae. At a conference, she connected with co-author Ylva Götberg, now an Assistant Professor at the Institute of Science and Technology Austria (ISTA), who had recently developed new theoretical models depicting the expected appearance of these stars.

Drout, Götberg, and their team orchestrated a novel survey tailored for the ultraviolet segment of the spectrum, where exceptionally hot stars predominantly emit their light. Although imperceptible to the naked eye, specialized instruments and telescopes can detect ultraviolet light.

Leveraging data from the Swift Ultra-Violet/Optical Telescope, the researchers compiled brightness data for millions of stars in the Large and Small Magellanic Clouds, which are among the nearest galaxies to Earth. Ludwig pioneered the creation of the first comprehensive UV catalogue of the Magellanic Clouds, employing UV photometry to identify systems exhibiting distinctive UV emissions, indicative of the potential presence of a stripped star.

Conducting a preliminary examination of 25 celestial objects, the researchers conducted optical spectroscopy using the Magellan Telescopes at Las Campanas Observatory from 2018 to 2022. Through these observations, they demonstrated that the stars exhibited characteristics aligning with their model predictions: they were hot, compact, hydrogen-deficient, and located in binary systems.

The ongoing research involves a thorough investigation of the stars highlighted in this study, with an expanded search extending to both nearby galaxies and our Milky Way. Approved programs on instruments such as the Hubble Space Telescope, the Chandra X-Ray Telescope, the Magellan Telescopes, and the Anglo-Australian Telescope are employed in this quest. As part of their commitment to transparency, all theoretical models and data utilized in identifying these stars have been disclosed and are accessible to the broader scientific community.