The evolution of triples with a Roche-lobe filling outer star

TL;DR

This paper investigates the evolution of triple star systems, focusing on cases where the outer star fills its Roche lobe, and uses detailed simulations to explore the complex mass transfer processes involved.

Contribution

It presents the first detailed simulations of Roche-lobe overflow in the outer star of triple systems, integrating stellar evolution, dynamics, and hydrodynamics using AMUSE.

Findings

Approximately 1% of triples may experience outer star Roche-lobe overflow.

Mass transfer significantly affects the inner and outer orbits.

Complex evolution during mass transfer makes outcomes difficult to predict.

Abstract

The evolution of triples has not attracted much attention in the literature, although their evolution can be dramatically different from binaries and single stars. Triples are quite common, and we find that for about 1% of the triples in the Tokovinin catalogue of multiple stellar systems in the solar neighbourhood, the tertiary star will overflow its Roche lobe at some time in its evolution, before any of the inner stars leave the main sequence. For two of these systems, Xi Tauri and HD97131 we simulate in detail this phase of mass transfer, during which stellar evolution, gravitational dynamics and hydrodynamics all play an important role. We have used the Astrophysical Multi-purpose Software Environment (AMUSE) to solve these physical processes in a self-consistent way. The resulting evolution, mass transfer and the effects on the inner as well as on the outer orbit are profound, although it is not trivial to predict the eventual consequence of the phase of mass transfer and the appearance of the resulting system.