Very massive runaway stars from three-body encounters

TL;DR

This paper demonstrates through simulations that three-body encounters involving very massive binaries can produce high-velocity runaway stars, explaining observations that binary-supernova scenarios cannot.

Contribution

It introduces a new dynamical mechanism for the origin of massive runaway stars, emphasizing the role of three-body encounters with very massive binaries.

Findings

Three-body encounters can produce runaway stars with velocities ≥70 km/s.

The ejection of stars is accompanied by binary recoil in the opposite direction.

The observed positions of R145 and Sk-69 206 support a common ejection event from R136.

Abstract

Very massive stars preferentially reside in the cores of their parent clusters and form binary or multiple systems. We study the role of tight very massive binaries in the origin of the field population of very massive stars. We performed numerical simulations of dynamical encounters between single (massive) stars and a very massive binary with parameters similar to those of the most massive known Galactic binaries, WR 20a and NGC 3603-A1. We found that these three-body encounters could be responsible for the origin of high peculiar velocities ($\geq$ 70 km/s) observed for some very massive ($\geq$ 60-70 Msun) runaway stars in the Milky Way and the Large Magellanic Cloud (e.g., $\lambda$ Cep, BD+43 3654, Sk-67 22, BI 237, 30 Dor 016), which can hardly be explained within the framework of the binary-supernova scenario. The production of high-velocity massive stars via three-body encounters is accompanied by the recoil of the binary in the opposite direction to the ejected star. We show that the relative position of the very massive binary R145 and the runaway early B-type star Sk-69 206 on the sky is consistent with the possibility that both objects were ejected from the central cluster, R136, of the star-forming region 30 Doradus via the same dynamical event -- a three-body encounter.