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Astronomers at the University of Toronto have discovered a population of massive stars that have been stripped of their outer hydrogen layer by companion stars.

For over a decade, scientists have theorized that approximately one in three massive stars are stripped of their hydrogen envelope in binary systems (systems where two stars are gravitationally bound to one another). Yet, until now, only one possible candidate had been identified.

The findings, , shed light on the hot helium stars that are believed to be the origins of hydrogen-poor core-collapse supernovae and neutron star mergers.

“If it turned out that these stars are rare, then our whole theoretical framework for all these different phenomena is wrong, with implications for supernovae, gravitational waves and the light from distant galaxies,” said Maria Drout, assistant professor in the  at the University of Toronto and an associate at the .

“This finding shows these stars really do exist.”

It also opens up possibilities for more detailed research going forward. “For example, predictions for how many neutron star mergers we should see are dependent on the properties of these stars, such as how much material comes off of them in stellar winds," Drout says. "Now, for the first time, we’ll be able to measure that, whereas people have been extrapolating it before."

Drout and her colleagues propose that these newly discovered stars will eventually explode as hydrogen-poor supernovae. These star systems are also thought to be necessary to form neutron star mergers.

In fact, the researchers believe that a few objects in their current sample are stripped stars with neutron star or blackhole companions. These objects are at the stage immediately before they become double-neutron-star or neutron-star-plus-blackhole systems that could eventually merge.

“Many stars are part of a cosmic dance with a partner, orbiting each other in a binary system. They’re not solitary giants but part of dynamic duos, interacting and influencing each other throughout their lifetimes,” says Bethany Ludwig, a PhD student in the David A. Dunlap department of Astronomy & Astrophysics and third author on the paper. “Our work sheds light on these fascinating relationships, revealing a universe that is far more interconnected and active than we previously imagined.

“Just as humans are social beings, stars too, especially the massive ones, are rarely alone.”

As stars evolve and expand to become red giants, the hydrogen at the outer edges of one can be stripped by the gravitational pull of its companion star – leaving a very hot helium core exposed. The process can take tens of thousands or even hundreds of thousands of years.

Stripped stars are difficult to find because much of the light they emit is outside of the visible light spectrum and can be obstructed by dust in the universe or outshone by their companion stars.

Drout and her collaborators began their search in 2016. Having studied hydrogen-poor supernovae during her PhD, Drout set out to find the stripped stars thought to be at the heart of these supernovae during a NASA Hubble postdoctoral fellowship at the Observatories of the Carnegie Institution for Science.

The researchers, who include co-author Ylva Götberg, assistant professor at the Institute of Science and Technology Austria, later designed a survey to look in the ultraviolet part of the spectrum where extremely hot stars emit most of their light. Using data from the Swift Ultra-Violet/Optical Telescope, they collected brightness data for millions of stars in the Large and Small Magellanic Clouds, two of the closest galaxies to Earth.

Ludwig, who developed the first wide-field UV catalogue of the Magellanic Clouds, used UV photometry to detect systems with unusual UV emissions – signaling the possible presence of a stripped star.

The team carried out a pilot study of 25 objects, obtaining optical spectroscopy with the Magellan Telescopes at Las Campanas Observatory between 2018 and 2022, and demonstrated that the stars were hot, small, hydrogen-poor, and in binary systems – all consistent with their model predictions.

Currently, the researchers are continuing to study the stars identified in the paper and expanding their search to find more. They will be looking both within our own Milky Way and nearby galaxies with approved programs on the Hubble Space Telescope, the Chandra X-ray Telescope, the Magellan Telescopes and the Anglo-Australian Telescope.

As part of this publication, all theoretical models and data used to identify these stars have been made public and available to other scientists.

Collaborating institutions include the University of Toronto, the Observatories of the Carnegie Institution for Science, Max-Planck-Institut für Astrophysik, Anton Pannekoek Institute for Astronomy, Dunlap Institute for Astronomy & Astrophysics and Steward Observatory.