The origin of B-type runaway stars based on kinematics

TL;DR

This study investigates the origins of 39 B-type runaway stars using kinematic analysis, spectral data, and machine learning to trace their trajectories and assess whether they originate from the Galactic disk or clusters.

Contribution

It combines orbital traceback, spectral analysis, and machine learning to identify the likely origins of B-type runaway stars, providing new insights into their kinematic properties and potential formation scenarios.

Findings

29 stars have trajectories within the Galactic disk

10 stars are disk-passing but still trace back to the disk

Some stars may originate from known clusters

Abstract

Runaway stars depart their birthplaces with high peculiar velocities. Two mechanisms are commonly invoked to explain their origin, the binary supernova scenario (BSS) and the dynamical ejection scenario (DES). Investigating the kinematic properties of runaway stars is key to understanding their origins.We intend to investigate the origins of 39 B-type runaway stars from LAMOST using orbital traceback analysis. From the catalog of LAMOST, we selected 39 B-type runaway stars and determined their spectral subtypes from key absorption lines. We then derived atmospheric parameters for each star using the Stellar Label Machine (SLAM), which is trained on TLUSTY synthetic spectra computed under the non-local thermodynamic equilibrium (NLTE) assumption. Using the derived atmospheric parameters as input, we estimated stellar masses and ages with a machine learning model trained on PARSEC evolutionary tracks. We finally performed orbital traceback with GALPY to analyze their origins. Through orbital traceback, we find that 29 stars have trajectories entirely within the Galactic disk, whereas 10 are disk-passing yet still trace back to the disk. Two stars have trajectories that intersect those of known clusters. Their orbits show similar morphologies in both the $X-Y$ and $R-Z$ planes, and their [M/H] values are comparable, suggesting possible cluster origins. However, definitive confirmation will require additional evidence. In addition, the $V_{\rm Sp} - v\sin{i}$ plane shows that runaway stars with low peculiar space velocities but high $v\sin{i}$ remain on the Galactic disk, whereas those with high peculiar space velocities but low $v\sin{i}$ pass through the disk, possibly reflecting two distinct origins.