In a groundbreaking discovery, astronomers at the University of Toronto have identified the first population of massive stars stripped of their outer hydrogen layers by companion stars, confirming a long-standing theory and offering new insights into the origins of hydrogen-poor core-collapse supernovae and neutron star mergers.
The findings, published in the prestigious journal Science, reveal the existence of hot helium stars believed to be at the core of critical cosmic events. For over a decade, scientists speculated that one in three massive stars undergoes hydrogen envelope stripping in binary systems, where two stars are gravitationally bound to each other. However, until now, only one potential candidate has been identified.
Assistant Professor Maria Drout, from the David A. Dunlap Department of Astronomy & Astrophysics at the University of Toronto, emphasizes the significance of this discovery: “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. This finding shows these stars really do exist.”
The researchers believe these stripped stars will eventually lead to hydrogen-poor supernovae and play a vital role in the formation of neutron star mergers. Some stars in the current sample are suspected to have neutron star or black hole companions, representing a stage just before becoming double-neutron-star or neutron-star-plus-black-hole systems that could eventually merge.
Bethany Ludwig, a PhD student in the David A. Dunlap Department of Astronomy & Astrophysics and co-author on the paper, reflects on the cosmic dance of binary star systems: “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 into red giants, gravitational forces from companion stars strip the hydrogen at their outer edges, leaving behind a hot helium core. This process, taking tens of thousands or even hundreds of thousands of years, results in the formation of stripped stars.
Identifying stripped stars poses a unique challenge due to the majority of emitted light falling outside the visible spectrum and potential obstruction by dust or overshadowing by companion stars. The research team, led by Drout, initiated their search in 2016, conducting a survey in the ultraviolet spectrum where extremely hot stars emit the majority of their light.
The team, including co-author Ylva Götberg, assistant professor at the Institute of Science and Technology Austria, carried out a pilot study of 25 objects, leveraging data from the Swift Ultra-Violet/Optical Telescope. Optical spectroscopy with the Magellan Telescopes at Las Campanas Observatory between 2018 and 2022 validated the stars as hot, small, hydrogen-poor, and in binary systems, aligning with model predictions.
The researchers continue their investigation of these stars, expanding their search within the Milky Way and nearby galaxies using instruments such as the Hubble Space Telescope, the Chandra X-ray Telescope, the Magellan Telescopes, and the Anglo-Australian Telescope.
As part of their commitment to advancing scientific knowledge, the team has made all theoretical models and data used in the identification of these stars publicly accessible to the scientific community.
Collaborating institutions on this groundbreaking research 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.
