The evolution of hierarchical triple star-systems

Abstract

Field stars are frequently formed in pairs, and many of these binaries are part of triples or even higher-order systems. Even though, the principles of single stellar evolution and binary evolution, have been accepted for a long time, the long-term evolution of stellar triples is poorly understood. The presence of a third star in an orbit around a binary system can significantly alter the evolution of those stars and the binary system. The rich dynamical behaviour in three-body systems can give rise to Lidov-Kozai cycles, in which the eccentricity of the inner orbit and the inclination between the inner and outer orbit vary periodically. In turn, this can lead to an enhancement of tidal effects (tidal friction), gravitational-wave emission and stellar interactions such as mass transfer and collisions. The lack of a self-consistent treatment of triple evolution, including both three-body dynamics as well as stellar evolution, hinders the systematic study and general understanding of the long-term evolution of triple systems. In this paper, we aim to address some of these hiatus, by discussing the dominant physical processes of hierarchical triple evolution, and presenting heuristic recipes for these processes. To improve our understanding on hierarchical stellar triples, these descriptions are implemented in a public source code TrES, which combines three-body dynamics (based on the secular approach) with stellar evolution and their mutual influences. Note that modelling through a phase of stable mass transfer in an eccentric orbit is currently not implemented in TrES, but can be implemented with the appropriate methodology at a later stage.

Binary Black Hole Mergers from Field Triples: Properties, Rates, and the Impact of Stellar Evolution

“…In this paper we have studied how the evolution of massive triples can lead to the formation of BH triple systems and how their subsequent dynamical evolution can result into the merger of two BHs. We study this problem using a combination of high-precision direct integrations (Mikkola & Merritt 2008) and a code that combines secular three-body dynamics with stellar evolution and their mutual influences (Toonen et al 2016). Our approach allows us to make reliable predictions about the properties of the newly formed BH triples, and about the rates and properties of the merging binaries.…”

“…To simulate the formation of triple BH systems, we use the recently published code TRES (Toonen et al 2016) for studying the evolution of coeval, dynamically stable, hierarchical, stellar triples. The simulations start with three stars on the zero-age main sequence in a specific orbital configuration.…”

“…In our simulations the orbital evolution of a triple due to an instantaneous supernova kick was modeled using the procedure described in the appendix of Toonen et al (2016). In summary, the positions and velocities of the three bodies before and just after the supernova are computed from the orbital elements assuming a random mean anomaly, and assuming no changes in the position vectors.…”

“…We use a numerical algorithm (Toonen et al 2016) that combines the secular triple dynamics with stellar evolution and interactions to follow the evolution of stellar triples and the way this results in the formation of BH triples. After their formation, the BH triples are evolved forward in time using either the octupole level secular equations of motion of triple systems (e.g., Blaes et al 2002;Naoz et al 2013) or a high-precision direct integrator for systems with weaker hierarchies where the secular equations become inaccurate (Mikkola & Merritt 2008).…”

Binary stars in the Galactic thick disc

“…It is possible that triple-star interactions lead to a similar number of interacting binaries even with an initial orbital period distribution more like that of the solar neighbourhood. However quantitative predictions are still rather preliminary (Toonen et al 2016).…”