Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution (1703.06614v2)

Abstract: We consider the formation of binary black hole mergers through the evolution of field massive triple stars. In this scenario, favorable conditions for the inspiral of a black hole binary are initiated by its gravitational interaction with a distant companion, rather than by a common-envelope phase invoked in standard binary evolution models. We use a code that follows self-consistently the evolution of massive triple stars, combining the secular triple dynamics (Lidov-Kozai cycles) with stellar evolution. After a black hole triple is formed, its dynamical evolution is computed using either the orbit-averaged equations of motion, or a high-precision direct integrator for triples with weaker hierarchies for which the secular perturbation theory breaks down. Most black hole mergers in our models are produced in the latter non-secular dynamical regime. We derive the properties of the merging binaries and compute a black hole merger rate in the range (0.3- 1.3) Gpc^{-3}yr^{-1}, or up to ~2.5Gpc^{-3}yr^{-1} if the black hole orbital planes have initially random orientation. Finally, we show that black hole mergers from the triple channel have significantly higher eccentricities than those formed through the evolution of massive binaries or in dense star clusters. Measured eccentricities could therefore be used to uniquely identify binary mergers formed through the evolution of triple stars. While our results suggest up to ~10 detections per year with Advanced-LIGO, the high eccentricities could render the merging binaries harder to detect with planned space based interferometers such as LISA.

• The paper introduces a model where gravitational perturbations in field triples trigger binary black hole mergers, bypassing the common-envelope phase.
• The paper employs a hybrid approach combining secular three-body dynamics with direct integration to simulate the intricate evolution of massive triple systems.
• The paper finds that mergers from field triples display high eccentricities, with estimated rates reaching up to 2.5 Gpc⁻³yr⁻¹, offering new detection prospects for gravitational wave observatories.

A Study on Binary Black Hole Mergers from Field Triples: Formation, Evolution, and Implications

The paper "Binary black hole mergers from field triples: properties, rates and the impact of stellar evolution" by Antonini et al. provides a thorough exploration of the formation and dynamics of binary black hole (BH) mergers originating from distant triple star systems in galactic fields. This research advances our understanding of black hole binaries' formation through stellar evolution and complex gravitational interactions involving a third stellar component, distinct from the more traditional binary evolution pathways typically considered.

Formation Scenario and Methodology

The authors propose a model in which the inspiral of a black hole binary in a triple system is primarily driven by gravitational perturbations from a distant tertiary companion. This mechanism circumvents the common-envelope phase often invoked in binary black hole formation scenarios. Central to their approach is the integration of secular three-body dynamics, specifically the Lidov-Kozai cycles, with stellar evolution processes. The paper employs a specialized computational code to simulate the evolution of massive triples with attention to both stellar interactions and dynamics until the point of black hole formation. Once a triple black hole system is established, its dynamics are modeled using either orbit-averaged motion equations or high-precision direct integration tailored for systems with weaker gravitational hierarchies, where the secular approximation fails.

Results and Key Findings

The research identifies that the majority of black hole mergers in triple systems result from a non-secular, chaotic dynamical regime, indicating that direct numerical simulations are necessary for accurate predictions. The authors calculate a merger rate of $(0.3-1.3) \, \rm Gpc^{-3}yr^{-1}$, potentially extending to $2.5\, \rm Gpc^{-3}yr^{-1}$ if initial black hole orbital planes are randomly oriented. An interesting result is the discovery that black hole mergers via the triple channel exhibit significantly higher eccentricities compared to those from isolated massive binaries or dense star clusters. Such characteristics could allow observational discrimination among merger origins based purely on eccentricity measurements.

Theoretical Implications

This paper introduces a sophisticated perspective on the role of triple systems in the evolution of black hole binaries, challenging standard binary-centric models. The use of direct integration techniques to handle systems where secular perturbation theory does not hold marks a significant methodological shift, highlighting the complex dynamical pathways leading to mergers. The documented high eccentricities in mergers suggest new parameter spaces that gravitational wave detectors like Advanced LIGO could explore to identify triple-system-derived black hole mergers.

Practical Implications and Future Directions

While predicting up to ten detectable events per year with Advanced LIGO, the research warns that the high eccentricities might impede the detectability of these mergers using space-based interferometers like LISA. The complex dynamical interactions, leading to non-circular merger precursors, presents a new frontier for gravitational wave astronomy, particularly in waveform modeling and data analysis aligned with such unique signatures.

The framework and insights provided by this paper on field triples set the stage for future work assessing the gravitational wave signals' detectability across different observational platforms. Moreover, subsequent research can extend these findings by exploring higher-order relativistic effects, diverse natal kick distributions, and incorporating stochastic background influences such as encounters with other field stars or massive objects.

In conclusion, Antonini et al. deliver a significant contribution to the field, enriching our understanding of the diverse formation channels of binary black holes. This work not only augments theoretical models of stellar evolution and black hole dynamics but also ignites new discussions on detection strategies that could influence the future trajectory of gravitational wave astronomy.