Researchers from the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC), in collaboration with the Institute of Astrophysics of the Canary Islands (IAC), have led the most extensive observational study to date of runaway massive stars, which includes an analysis of the rotation and binarity of these stars in our galaxy. This study, published today in the journal
Astronomy & Astrophysics, sheds new light on how these stellar “runaways” are ejected into space and what their properties reveal about their fascinating origins.
Runaway stars are stars that move through space at unusually high speeds, drifting away from the places where they were born. For a long time, the way massive runaway stars acquire these high velocities has remained a mystery to astronomers, who have considered two main scenarios: a violent kick when a companion explodes as a supernova in a binary system, or a gravitational ejection during close encounters in dense, young star clusters. However, the relative contribution of these scenarios to explaining massive runaway stars in the Milky Way was not well understood.
Using data from the Gaia mission of the European Space Agency (ESA) and high-quality spectroscopic information from the IACOB project, the team analyzed 214 O-type stars, the most massive and luminous stellar objects in the galaxy. To understand their origins, they combined measurements of rotation speed and binarity (whether the star is single or part of a binary system) for the largest sample of galactic O-type runaway stars to date.
At what speed do runaway stars move and rotate?
The results reveal that most runaway stars rotate slowly, but those that rotate faster are more likely to be linked to supernova explosions in binary systems. Stars with the highest space velocities tend to be single, suggesting that they were ejected from young clusters through gravitational interactions.
It was also found that there are almost no runaway stars that both move and rotate rapidly, highlighting the possibility of different formation pathways.
The team also identified 12 runaway binary systems, including three known high-mass X-ray binaries (systems hosting neutron stars or black holes) and three additional binary systems that are promising candidates to host black holes.
Massive runaway stars are not just astronomical curiosities: they influence the evolution of galaxies. By escaping their birthplaces, they disperse heavy elements and radiation into the interstellar medium, shaping future generations of stars and planets. Understanding their origins helps refine models of stellar evolution, supernova explosions, and even the formation of gravitational wave sources. In this context, this study serves as a reference for the next generation of models of massive binary system evolution and dynamical studies of star clusters.
“This is the most comprehensive observational study of its kind in the Milky Way,” says researcher Mar Carretero-Castrillo, member of the ICCUB and IEEC, lead author of the study and currently at the European Southern Observatory. “By combining information on rotation and binarity, we provide the community with unprecedented constraints on how these runaway stars are formed.”
Future Gaia data releases and ongoing spectroscopic studies will allow astronomers to expand these samples and trace the past trajectories of runaway stars, linking them to their birthplaces. This will help confirm which formation mechanisms dominate and uncover new candidates for exotic systems, such as high-energy binary systems hosting neutron stars or black hole companions.