On the origin of high-velocity runaway stars

TL;DR

This paper investigates how high-velocity runaway stars can be produced through exchange encounters between massive binaries and very massive stars, showing they can reach velocities high enough to escape the galaxy.

Contribution

It demonstrates that such exchange encounters can produce high-velocity stars with speeds comparable to pulsars and halo stars, providing a new mechanism for their origin.

Findings

Ejected stars can reach velocities of 200-400 km/s.

Stars of 3-4 Msun can attain velocities of 300-400 km/s.

Some stars can exceed the Milky Way's escape velocity.

Abstract

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary 50-100 Msun star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of $\ga$ 200-400 km/s (typical of pulsars), while 3-4 Msun stars can attain velocities of $\ga$ 300-400 km/s (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.