Binary imposters: Mergers in massive hierarchical triple stars

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
² Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^⋆ Corresponding author: f.a.kummer@uva.nl

Received: 1 August 2025
Accepted: 11 September 2025

Abstract

Context. Massive stars are often born in triples, where gravitational dynamics and stellar interactions play a crucial role in shaping their evolution. One such pathway includes the merger of the inner binary, transforming the system into a binary with a distinct formation history. Therefore, the interpretation of observed binary properties and their inferred formation history may require the consideration of a potential triple origin.

Aims. We aim to investigate the population of stellar mergers in massive hierarchical triples. Specifically, we assess how frequently mergers occur and characterise the properties of the post-merger binaries and their subsequent evolution.

Methods. We combined the triple population synthesis code TRES, which self-consistently models stellar evolution, binary interaction, and gravitational dynamics with the binary population synthesis code SeBa to simulate 10⁵ dynamically stable, massive triples from the zero-age main sequence through merger and post-merger evolution. We explored the effects of a range of physical models for the initial stellar properties, mass transfer, and merger.

Results. We find that stellar mergers are a common outcome, occurring in 20–32% of massive triples. Most mergers happen relatively early in the evolution of the system and involve two main-sequence (MS) stars, producing rejuvenated merger remnants that can appear significantly younger than their tertiary companions. Consequently, we predict that 2–10% of all wide MS+MS binaries (P >  100 days) have a measurable age discrepancy, and serve as a promising way to identify merged stars. The post-merger systems preferentially evolve into wide, eccentric binaries, with ∼80% avoiding further interaction. However, a notable fraction (16–22%) undergoes a second mass-transfer phase, which may result in the formation of high-mass X-ray binaries or mergers of compact objects that spiral in via gravitational-wave emission. Our results highlight the crucial role that stellar mergers in triples play in shaping the population of massive binary stars.