The evolution of stellar triples

The most common evolutionary pathways

¹ Institute of Gravitational Wave Astronomy, School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
e-mail: toonen@star.sr.bham.ac.uk
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Institute for Advanced Study, School of Natural Sciences, Einstein Drive, Princeton, NJ 08540, USA
⁵ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany

Received: 3 October 2019
Accepted: 8 May 2020

Abstract

Context. Many stars do not live alone, but instead have one or more stellar companions. Observations show that these binaries, triples, and higher-order multiples are common. While the evolution of single stars and binaries have been studied extensively, the same is not true for the evolution of stellar triples.

Aims. To fill in this gap in our general understanding of stellar lives, we aim to systematically explore the long-term evolution of triples and to map out the most common evolutionary pathways that triples go through. We quantitatively study how triples evolve, which processes are the most relevant, and how this differs from binary evoluion.

Methods. We simulated the evolution of several large populations of triples with a population synthesis approach. We made use of the triple evolution code TRES to simulate the evolution of each triple in a consistent way, including three-body dynamics (based on the secular approach), stellar evolution, and their mutual influences. We simulated the evolution of the system up until mass transfer starts, the system becomes dynamically unstable, or a Hubble time has passed.

Results. We find that stellar interactions are common in triples. Compared to a binary population, we find that the fraction of systems that can undergo mass transfer is ∼2−3 times larger in triples. Moreover, while orbits typically reach circularisation before Roche-lobe overflow in binaries, this is no longer true in triples. In our simulations, about 40% of systems retain an eccentric orbit. Additionally, we discuss various channels of triple evolution in detail, such as those where the secondary or the tertiary is the first star to initiate a mass transfer event.