A research team from the Yunnan Observatories of the Chinese Academy of Sciences (CAS), in collaboration with researchers from the CAS National Astronomical Observatories and Hebei Normal University, has revealed the origin of B-type runaway stars based on kinematic analysis. Their findings were recently published in Astronomy & Astrophysics.

This study conducted orbital traceback analyses for a sample of 39 B-type runaway stars and identified a correspondence between the traced trajectories and the two proposed formation channels, providing new observational evidence to constrain the origins of such objects.

Runaway stars are characterized by their high peculiar velocities, typically exceeding 30–40 km s^-1. Two distinct mechanisms have been proposed to explain their origin: the dynamical ejection scenario (DES), in which stars are ejected via close dynamical encounters in dense stellar systems, and the binary-supernova scenario (BSS), in which a star is released from a close binary system after its companion explodes as a supernova. Runaway stars originating from DES and BSS can be distinguished by tracing their orbits back to star clusters, examining their chemical signatures, and comparing their kinematic properties. DES runaway stars generally exhibit higher space velocities, whereas stars with high projected rotational velocities (v sin i) are thought to originate primarily from the BSS channel.

In this study, the team selected 39 B-type runaway stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) catalog. They determined spectral subtypes from key absorption lines and derived atmospheric parameters using the Stellar Label Machine (SLAM), which was trained on TLUSTY synthetic spectra computed under the non-local thermodynamic equilibrium (NLTE) assumption. These atmospheric parameters were then used to estimate stellar masses and ages with a machine-learning model trained on the PAdova and TRieste Stellar Evolution Code (PARSEC) evolutionary tracks. Finally, orbital traceback analysis was performed using the Galactic Dynamics PYthon Library (GALPY).

The analysis revealed that 29 stars have trajectories confined entirely within the Galactic disk, while 10 pass through the disk but can still be traced back to it. Two stars have trajectories that intersect those of known star clusters.

In addition, distributions in the peculiar space velocity (V_Sp) versus projected rotational velocity (v sin i) plane suggest that runaway stars with low peculiar space velocities but high v sin i remain within the Galactic disk, whereas those with high peculiar space velocities but low v sin i pass through the disk. This pattern may reflect two distinct formation origins.

The researchers noted that this work provides new kinematic evidence to distinguish between the binary-supernova and dynamical ejection scenarios. However, the conclusions are limited by the lack of high-resolution spectroscopic abundance data.