Science & Technology, Canada (Commonwealth Union) – A group of astronomers from the University of Toronto has unveiled a population of massive stars undergoing a unique process: their outer hydrogen layers are being stripped away by companion stars.
For more than a decade, scientists have postulated that around one-third of massive stars experience the removal of their hydrogen envelope within binary systems—systems where two stars are gravitationally linked. However, until now, only one potential candidate had been pinpointed.
The research, featured in the journal Science, provides insights into hot helium stars thought to be the precursors of hydrogen-poor core-collapse supernovae and neutron star mergers.
Maria Drout, an assistant professor in the David A. Dunlap Department of Astronomy & Astrophysics at the University of Toronto and an associate at the Dunlap Institute for Astronomy & Astrophysics, emphasized the significance of the findings, indicating that if it turned out that these stars are rare, then our entire theoretical framework for all these different phenomena is wrong, with implications for supernovae, gravitational waves, and the light from distant galaxies.
“This finding shows these stars really do exist.”
Moreover, this discovery paves the way for more comprehensive research in the future. Drout explained that for example, predictions for how many neutron star mergers that should be seen are dependent on the properties of these stars, such as how much material comes off of them in stellar winds. Now, for the first time, they are able to measure that, whereas people have been extrapolating it on prior occasions.
Drout and her colleagues posit that these recently uncovered stars will inevitably undergo hydrogen-poor supernovae explosions. These star systems are also deemed crucial for the formation of neutron star mergers.
In fact, the researchers suggest that a few entities in their current sample are stripped stars paired with neutron stars or black hole companions. These entities are in the phase just before evolving into double-neutron-star or neutron-star-plus-black-hole systems that could eventually undergo mergers.
“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,” added Bethany Ludwig, a PhD student in the David A. Dunlap department of Astronomy & Astrophysics as well as the 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 undergo evolution and expand into red giants, the gravitational pull of a companion star can strip away the outer hydrogen layers of one star, revealing a highly heated helium core. This transformative process unfolds over tens of thousands or even hundreds of thousands of years, according to the researchers of the study.
Detecting stripped stars poses a challenge due to the fact that a significant portion of the emitted light falls beyond the visible light spectrum, obstructed by cosmic dust or overshadowed by the brilliance of their companion stars.
In 2016, Drout and her collaborators initiated their quest. Drawing on her expertise in hydrogen-poor supernovae gained during her Ph.D., Drout, while on a NASA Hubble postdoctoral fellowship at the Observatories of the Carnegie Institution for Science, embarked on locating the stripped stars believed to be central to these supernovae.
The research team, which includes co-author Ylva Götberg, an assistant professor at the Institute of Science and Technology Austria, devised a survey targeting the ultraviolet segment of the spectrum, where exceptionally hot stars emit the majority of their light. Leveraging data from the Swift Ultra-Violet/Optical Telescope, they gathered brightness information for millions of stars in the Large and Small Magellanic Clouds, two of the nearest galaxies to Earth.
