Massive stellar triples on the edge

A numerical study of the evolution and final outcomes of destabilised massive triples

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE, Amsterdam, The Netherlands
² Rudolf Peierls Centre for Theoretical Physics, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK
³ NASA Ames Research Center, Moffett Field, 94035 CA, USA

^⋆ Corresponding author; c.w.bruenech@uva.nl

Received: 16 August 2024
Accepted: 19 October 2024

Abstract

Context. 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.

Aims. We intend to provide a statistical overview of the lives and final outcomes of massive triples that are born dynamically stable but become unstable due to evolutionary processes.

Methods. We evolved a population of initially stable triples with a massive primary star from the zero-age main sequence (ZAMS) using the code TRES, which combines stellar evolution (SE) with orbit-averaged dynamics. The triples that become unstable were transferred to a direct N-body code, where they were simulated until the system disintegrated. This excludes systems undergoing mass transfer, where the instability is caused by stellar winds or supernova explosions. We performed two suites of N-body simulations; one with gravity as the only interaction, and one with SE included.

Results. We find that our triples remain on the edge of stability for a long time before disintegrating, making SE a consequential process during this phase. Eventually, the destabilisation results in either the ejection of a stellar body or the collision between two components. We find that collisions occur in 35 − 40% of systems, with the variation in this percentage coming from whether or not SE is included. The collisions predominantly involve two main sequence (MS) stars (70 − 78%) or a MS and post-MS star (13 − 28%). We estimate the Galactic rate of collisions due to massive triple destabilisation to be at 1.1 − 1.3 events per million years. Furthermore, we find that the process of destabilisation often ends in the ejection of one of the stellar bodies, specifically for 31 − 40% of systems. The ejected bodies have typical velocities of ∼6 km/s, with a tail stretching to 10² km/s. If we make the assumption that 20% of massive stars are runaway stars, then 0.1% of runaways originate from triple destabilisation. 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.