[Paper Review] Massive Stellar Triples Leading to Sequential Binary Black-Hole Mergers in the Field

Alejandro Vigna-Gómez, Silvia Toonen+4 more|arXiv (Cornell University)|Oct 26, 2020

Pulsars and Gravitational Waves ResearchPhysics and Astronomy93 references81 citations

TL;DR

This paper proposes that massive stellar triples in the field can produce sequential binary black-hole mergers, with the first merger forming a black hole that later merges with a tertiary black hole. Using population synthesis models, it shows that such systems can produce black holes in the pair-instability mass gap with effective spins >0.1, offering a field-origin alternative to cluster formation. The model supports GW170729 as a triple-origin event but suggests GW190521 likely formed in a cluster due to spin and mass inconsistencies.

ABSTRACT

Stellar triples with massive stellar components are common, and can lead to sequential binary black-hole mergers. Here, we outline the evolution towards these sequential mergers, and explore these events in the context of gravitational-wave astronomy and the pair-instability mass gap. We find that binary black-hole mergers in the pair-instability mass gap can be of triple origin and therefore are not exclusively formed in dense dynamical environments. We discuss the sequential merger scenario in the context of the most massive gravitational-wave sources detected to date: GW170729 and GW190521. We propose that the progenitor of GW170729 is a low-metallicity field triple. We support the premise that GW190521 could not have been formed in the field. We conclude that triple stellar evolution is fundamental in the understanding of gravitational-wave sources, and likely, other energetic transientsas well.

Motivation & Objective

• To investigate whether isolated massive stellar triples can produce sequential binary black-hole mergers leading to black holes in the pair-instability mass gap.
• To determine if such field-origin mergers can explain the most massive gravitational-wave sources detected, such as GW170729 and GW190521.
• To distinguish field triple origins from cluster origins using spin and mass signatures of black hole mergers.
• To assess the role of triple systems in populating the pair-instability mass gap without requiring dense dynamical environments.

Proposed method

• Modeling isolated hierarchical triple systems with circular, coplanar, prograde orbits, assuming dynamical stability via the critical mass ratio criterion.
• Using synthetic binary population synthesis (COMPAS) to generate initial mass distributions and black hole masses, incorporating mass loss, mass transfer, and chemically homogeneous evolution.
• Applying gravitational wave-driven inspiral times (Peters 1964) to estimate merger timescales for the outer binary after the inner BBH merger.
• Calculating post-merger orbital changes due to radiated mass and recoil kicks, assuming negligible kick for high-mass-ratio inner binaries.
• Computing effective spin (χeff) of the final merger as a function of inner binary and tertiary black hole spins and masses.
• Estimating sequential merger rates at z=0 using the BBH merger rate from Riley et al. (2020), yielding R < 3 Gpc⁻³ yr⁻¹.

Experimental results

Research questions

• RQ1 Can isolated massive stellar triples produce sequential binary black-hole mergers that result in black holes within the pair-instability mass gap?
• RQ2 Can such field-origin mergers explain the observed masses and spins of GW170729 and GW190521?
• RQ3 What spin and mass signatures distinguish sequential mergers from cluster-origin mergers in the mass gap?
• RQ4 How do radiated mass and recoil kicks from the first merger affect the survival and evolution of the outer binary?
• RQ5 What is the expected rate of sequential mergers from field triples, and how does it compare to cluster-based formation?

Key findings

• The model predicts that field-origin sequential mergers can produce black holes in the pair-instability mass gap, challenging the assumption that such events are exclusively cluster-formed.
• Systems with a tertiary black hole above the mass gap and χeff between 0.1 and 0.27 (red sub-region A) are viable field triple candidates.
• Systems with a tertiary below the mass gap and χeff between 0.38 and 0.58 (blue sub-region B) are also consistent with field triple evolution.
• The effective spin of GW170729 (χeff ≈ 0.25) is consistent with a triple-origin scenario, particularly sub-region B, supporting its field triple origin.
• The high mass and spin of GW190521 are inconsistent with the model’s predictions, suggesting a cluster origin for this event.
• The estimated sequential merger rate from field triples is R < 3 Gpc⁻³ yr⁻¹, comparable to rates from hierarchical triples in clusters and the field.