The Main Evolutionary Pathways of Massive Hierarchical Triple Stars

TL;DR

This study investigates the main evolutionary pathways of massive hierarchical triple star systems, highlighting the significant role of three-body dynamics and interactions in their evolution and the formation of compact-object systems.

Contribution

The paper introduces detailed simulations of massive triple star evolution, emphasizing the impact of third-star interactions and stability constraints on evolutionary outcomes.

Findings

Mass transfer occurs in 65-77% of systems, often with unevolved donors.

Triple interactions significantly influence stellar evolution compared to binary systems.

Ignoring three-body dynamics may overestimate compact-object merger rates.

Abstract

So far, stellar population studies have mainly focused on the evolution of single and binary stars. Recent observations show that triple and higher order multiple star systems are common, especially among massive stars. Introducing three-body dynamical effects can influence the evolution of an individual stellar system and hence affect the predicted rates of astrophysical sources that are a product of stellar evolution. We aim to constrain the main evolutionary pathways of massive hierarchical triple star systems and to quantify the effect of the third star on the evolution of the system. We model the massive triple star population by performing simulations of triple star evolution with the TRES code, which combines stellar evolution with secular evolution of triple systems, and explore how robust the predictions of these simulations are under variations of uncertain initial conditions. We find that interactions are common among massive triple stars. The majority of systems (65-77\%) experience a phase of mass transfer in the inner binary, often with an unevolved donor star. This differs significantly from isolated binary evolution, where mass transfer is less frequent (52.3\% instead of 67\% for our fiducial model) and the donors are typically post-main sequence stars. Initial constraints for dynamical stability as well as eccentricity oscillations driven by the third body facilitate the occurrence of interactions, for instance mass transfer. The requirement of dynamical stability at formation places quite stringent constraints on allowed orbital properties, reducing uncertainties in triple evolution that resort from these initial conditions. Ignoring three-body dynamics during evolution of non-interacting triples leads to triple compact-object systems with stronger eccentricity oscillations and thereby likely over-predicts the merger rate of compact objects in such systems.