A newly published study offers the most extensive look yet at the origins of massive “runaway” stars within the Milky Way.Researchers analyzed data from over 200 O-type stars – some of the brightest and most massive in our galaxy – to better understand how these stellar outliers are ejected from their birthplaces. The findings, published in Astronomy & Astrophysics, distinguish between scenarios involving supernova explosions and gravitational interactions, refining our understanding of stellar evolution and galactic dynamics.
A new study has revealed key insights into the origins of massive “runaway” stars within the Milky Way galaxy. Researchers from the Institute of Astrophysics of the Canary Islands (IAC), in collaboration with the Institute of Cosmos Sciences of the University of Barcelona (ICCUB) and the Institute of Space Studies of Catalonia (IEEC), conducted the largest observational study to date of these high-velocity stars, incorporating analysis of their rotation and whether they exist as binary systems.
Published recently in Astronomy & Astrophysics, the research sheds new light on the mechanisms that eject these stars and what their physical properties reveal about their origins. Understanding these processes is crucial for refining models of stellar evolution and galactic dynamics.
Runaway stars are defined by their unusually high velocities as they travel through space, moving away from their birthplaces. Astronomers have long debated how massive runaway stars acquire such speeds, with two leading hypotheses: a powerful kick from a supernova explosion when a companion star dies in a binary system, or a gravitational ejection during close encounters in dense, young star clusters. However, the relative contribution of these scenarios to the formation of massive runaway stars in the Milky Way remained unclear.
The team analyzed 214 O-type stars – the most massive and luminous objects in the galaxy – using data from Gaia, a space observatory operated by the European Space Agency (ESA). They combined this with high-quality spectroscopic observations from the IAC-led IACOB project. By combining measurements of rotational speed and whether the star is part of a binary system, researchers created the largest sample of Milky Way O-type runaway stars to date.
The results indicate that most runaway stars rotate slowly, but those that spin faster are often linked to supernova explosions in binary systems. Faster-moving stars, on the other hand, are typically solitary, suggesting they were ejected from young clusters through gravitational interactions. This finding provides a clearer picture of the different pathways that can create these stellar outliers.
Interestingly, the researchers found very few runaway stars that are both fast-moving and rapidly rotating, highlighting potential distinct formation pathways. The scientific team also identified twelve runaway binary systems, including three known high-mass X-ray binaries (systems containing neutron stars or black holes) and three additional promising candidates for harboring black holes.
Massive runaway stars aren’t merely astronomical curiosities; they significantly influence galactic evolution. As they escape their birthplaces, they disperse heavy elements and radiation throughout interstellar space, shaping future generations of stars and planets.
“This is the most comprehensive observational study of its kind in the Milky Way,” says Mar Carretero-Castrillo, the study’s lead author, who is currently working at the European Southern Observatory (ESO). “By combining information about rotation and binary status, we provide the scientific community with unprecedented constraints on how these runaway stars form,” adds Sergio Simón-Díaz, an IAC researcher and leader of the IACOB project.
Future data releases from Gaia and ongoing spectroscopic studies will allow for expanding these samples and tracing the past trajectories of runaway stars, linking them back to their origins. This will help confirm which formation mechanisms are dominant and potentially uncover new candidates for exotic systems, such as high-energy binaries containing neutron stars or black hole companions.
