Massive stellar triples on the edge: A numerical study of the evolution and final outcomes of destabilized massive triples

TL;DR

This study uses numerical simulations to analyze the evolution and outcomes of destabilized massive triple star systems, revealing collision rates, ejection velocities, and their potential contribution to runaway stars.

Contribution

It provides the first statistical overview of the final outcomes of destabilized massive triples, including collision rates and ejection velocities, using combined stellar evolution and N-body simulations.

Findings

Collision occurrence in 35-40% of systems

Ejected bodies have velocities around 6 km/s

Destabilization affects about 2% of massive triples

Abstract

Massive stars reside predominantly in triples or higher-order multiples. Their lives can be significantly affected by three-body interactions, making it an important area of study in the context of massive star evolution. In this study we provide a statistical overview of the lives and final outcomes of destabilized massive triples. A population of initially stable triples with a massive primary star are evolved from the zero-age main sequence using the code TRES, which combines stellar evolution with orbit-averaged dynamics. The triples that become unstable are transferred to a direct N-body code where they are simulated until the system disintegrates. This excludes systems undergoing mass transfer, such that the instability is caused by stellar winds or supernovae. Two suites of N-body simulations are performed; one with gravity as the only interaction, and one with stellar evolution included. We find that collisions occur in 35 - 40% of systems, with the variation coming from whether stellar evolution is included. The collisions mainly involve two main sequence stars (70 - 78%) or a main sequence and post-main sequence star (13 - 28%). We estimate a Galactic rate of collisions due to massive triple destabilization at 1.1 - 1.3 events per Myr. Furthermore, we find that the process of destabilization often ends in the ejection of one of the stellar bodies, specifically for 31 - 40% of systems. The ejected bodies have typical velocities of around 6 km/s, with a tail stretching to 102 km/s. If we assume that 20% of massive stars are runaway stars, then 0.1% of runaways originate from triple destabilization. Overall, our simulations show that triple instability affects approximately 2% of massive triples. However, we estimate that up to ten times as many systems can become unstable due to mass transfer in the inner binary, and these system may end up ejecting bodies at higher velocities.