Astronomers at the University of Toronto have discovered a population of massive stars that have been stripped of their hydrogen shells by their companions in binary systems. The results were published on: ScienceIt sheds light on hot helium stars, which are thought to be the source of hydrogen-poor supernova core collapses and neutron star mergers.

For more than a decade, scientists assumed that approximately one in three massive stars in binary systems lacked a hydrogen envelope. However, only one candidate has been identified so far.

“This was a very large, conspicuous hole,” says co-author Maria Draut, the David A. Dunlap Associate Professor of Astronomy and Astrophysics and faculty member at the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto. “If it turns out that these stars are rare, all of our theoretical foundations for all these different phenomena are wrong, including inferences about supernovae, gravitational waves, and light from distant galaxies,” says Draut. “This finding shows that these stars really exist.”

“We will be able to do much more detailed physics studies with these stars in the future,” says Draut. “For example, estimates of how many neutron star mergers we should observe depend on the properties of stars, such as how much material is ejected from them in stellar winds. Now, for the first time, we will be able to measure this, whereas people had predicted this before.”

Double fission stars have previously been used to explain why one-third of core-collapse supernovae contain much less hydrogen than a typical red supergiant star explosion. Draut and his colleagues suggest that these newly discovered stars will eventually explode as hydrogen-poor supernovae. These star systems are also thought to be necessary for the formation of merging neutron stars, similar to those emitting gravitational waves detected on Earth by the LIGO experiment.

In fact, researchers believe that many objects in the current samples are neutron stars or black holes. These objects are in the stage just before they become neutron star binaries, or systems of neutron stars and black holes that may eventually merge.

“Many stars are part of a cosmic dance with a partner orbiting each other in a binary system. “They are not lone giants, but part of dynamic duos that interact and influence each other throughout their lives,” says Ph.D. Bethany Ludwig. David A. Dunlap is a PhD student in the Department of Astronomy and Astrophysics at the University of Toronto and the third author of this paper. “Our work sheds light on these fascinating relationships and reveals a universe that is much more interconnected and active than we had previously imagined.”

“Just as humans are social creatures, stars, especially massive ones, are rarely solitary,” says Ludwig.

As stars evolve and expand to become red giants, the hydrogen at the outer edges of one of them can be stripped away from the gravitational pull of its companion, leaving a very hot helium core exposed. The process can take tens of thousands or even hundreds of thousands of years. Truncated stars are difficult to find because most of the light they emit is outside the visible light spectrum and may be blocked by dust in the universe or by satellite stars.