Prompt stellar and binary black hole mergers in tight triples

Insights from chemically homogeneous evolution

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
² School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
³ OzGrav: Australian Research Council Centre of Excellence for Gravitational Wave Discovery, Clayton, VIC 3800, Australia
⁴ Institute for Astronomy, School of Physics, Zhejiang University, 310058 Hangzhou, China
⁵ Center for Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA
⁶ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁷ National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^⋆ Corresponding author: avigna@mpa-garching.mpg.de

Received: 21 March 2025
Accepted: 4 June 2025

Abstract

Short-period massive binary stars are predicted to undergo a chemically homogeneous evolution (CHE), making them prime candidates for producing binary black holes (BBHs) that have the potential to merge within the age of the Universe. Most of these binaries have a tertiary companion and here we explore how a nearby third body could possibly influence this evolutionary channel. Our analysis combines analytic treatments of triple dynamics with insights from detailed stellar evolution models, focusing on the role of the von Zeipel-Lidov-Kozai mechanism, while also accounting for tidal and general relativistic apsidal precession. We examine the dynamics of triples at three critical evolutionary stages: the zero-age main sequence, shortly after the main sequence, and at the time of BBH formation. We find that, for triples with outer orbital periods less than 70 d(120 d), the inner binary can merge during(or after) the main sequence stage, leading to a hydrogen-rich (helium-rich) stellar merger. If a stellar merger is avoided, the inner binary could eventually form a BBH. In mildly hierarchical triples, with outer periods of around 100 d, the tertiary component can trigger a rapid merger of the BBH on timescales comparable to the outer orbital period. Stellar tides play a crucial role in determining the fate of the inner binary in such tight triple systems, as they can suppress the perturbative effects of the third star. When tidal forces damp the oscillations induced by the tertiary, the BBH merger may occur soon after stellar collapse. Notably, these outcomes are not restricted to CHE binaries but they can also be applied to any BBH formed from stars in tight orbits. Mergers in these systems are characterised by the proximity of a tertiary companion and the presence of recently ejected gas, making them promising candidates for electromagnetic counterparts and gravitational wave signals influenced by nearby tertiary objects.