Runaway stars are stars that travel through space at unusually high speeds and gradually drift away from the regions where they formed. For decades, astronomers have debated how massive runaway stars gain these high velocities, generally favoring two main scenarios. In one case, a star receives a strong kick when its companion in a binary system explodes as a supernova. In the other, the star is flung out of a dense, young star cluster through strong gravitational encounters with other massive stars.

Until now, the relative importance of these two mechanisms in producing runaway O-type stars in the Milky Way has remained poorly constrained. To tackle this problem, the team used high-precision astrometric data from the European Space Agency's Gaia mission together with high-quality spectroscopic observations from the IACOB project. This combination allowed them to measure both the space motions and the rotation speeds of the stars, as well as to identify which objects are in binary systems.

The study analyzed 214 O-type stars, building the largest sample of galactic O-type runaway stars with combined information on rotation and binarity. The researchers derived the stars' space velocities from Gaia data and obtained their projected rotational velocities and binarity status from the spectroscopic observations. By bringing these measurements together, they were able to link different runaway properties to specific formation pathways.

The results show that most runaway stars rotate slowly, indicating that high space velocity does not necessarily go hand in hand with rapid rotation. However, the subset of runaway stars that do rotate more quickly is more likely to be associated with the supernova scenario, in which a star is spun up and then kicked when its binary companion explodes. This connection suggests that stellar rotation carries a clear imprint of a past binary interaction and supernova event.

The analysis also reveals that the fastest runaway stars in terms of space velocity are typically single, supporting the idea that they were ejected from young clusters through gravitational interactions rather than by a supernova in a binary system. In these crowded cluster environments, close multi-body encounters can efficiently accelerate massive stars and hurl them out into the galactic field. The near absence of very fast, rapidly rotating runaways points to distinct, mutually exclusive formation channels.

Another notable outcome of the study is the identification of 12 runaway binary systems among the sample. Three of these are already known high-mass X-ray binaries, which host compact objects such as neutron stars or black holes accreting matter from a massive companion. The remaining three systems stand out as promising new candidates for hosting black holes, making them particularly interesting targets for follow-up observations across multiple wavelengths.

Massive runaway stars play an important role in the evolution of galaxies because they carry energy and heavy elements away from their birthplaces and into more remote regions of the interstellar medium. As they move, they inject radiation, stellar winds, and, ultimately, supernova ejecta into gas that might otherwise remain relatively undisturbed. This dispersal can influence how and where new generations of stars and planets form and affects the chemical and dynamical structure of the galactic environment.

By clarifying how different ejection mechanisms operate and how often they occur, the new work provides tighter constraints on models of massive binary evolution, supernova explosions, and the internal dynamics of young star clusters. These insights extend to the study of gravitational wave sources, since many compact-object binaries detected via gravitational waves are thought to originate from massive stellar binaries shaped by similar physical processes. Understanding which pathways dominate for runaway stars helps refine scenarios for how compact binaries form and merge.

Lead author Mar Carretero-Castrillo, a member of ICCUB and IEEC and currently at the European Southern Observatory, emphasizes the scope of the project. She describes it as "the most comprehensive observational study of its kind in the Milky Way" and notes that combining rotational and binarity information provides "unprecedented constraints" on how runaway stars form. This integrated approach goes beyond earlier studies that focused solely on kinematics or limited binary diagnostics.

The authors expect that future data releases from Gaia, together with ongoing and planned spectroscopic surveys, will expand the sample of known runaway massive stars and improve the accuracy of their orbital histories. With more precise measurements, astronomers will be able to trace these stars back to their likely birth clusters or binary companions, directly testing which ejection scenarios apply in individual cases. This, in turn, will help identify new high-energy binary systems that may host neutron stars or black holes.

As observational capabilities continue to advance, studies like this one will deepen the connection between detailed stellar astrophysics and large-scale galactic evolution. Massive runaway stars serve as probes of both their turbulent birth environments and the violent endpoints of stellar evolution. Mapping their properties across the Milky Way is an essential step toward a more complete picture of how massive stars live, die, and shape the cosmos.

Research Report:An observational study of rotation and binarity of Galactic O-type runaway stars

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