Detailed simulations of massive hierarchical triple star systems

Exploring the impact of the stellar physics on the evolutionary pathways of massive hierarchical triple systems

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, CH-1290 Versoix, Switzerland
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^⋆ Corresponding author: luca.sciarini@unige.ch

Received: 29 January 2025
Accepted: 23 April 2025

Abstract

Context. Recent observations estimate that approximately 30% of early B- and O-type stars are found in triple systems. To date, the evolution of triple star systems has mainly been modeled using fast stellar codes. The accuracy of these codes is expected to decrease with increasing mass, which limits their reliability for predicting the evolutionary pathways of massive hierarchical triple star systems.

Aims. Our aim was to investigate the discrepancies in the predicted evolution of massive stars between fast (Hurley tracks) codes and detailed (MESA) codes, and to assess how these differences can impact the evolutionary pathways of massive triple star systems.

Methods. We coupled the TRES code, which by default uses SEBA to MESA to perform the first simulations of triple systems that combine a triple secular evolutionary code with a detailed, on-the-fly stellar code. After examining the differences between the single star evolution predicted by the two stellar codes (MESA and SEBA), we simulate the evolution of a set of triple systems and compare their predicted evolutionary pathways under identical initial conditions.

Results. We show that among very massive stars (M ≥ 50 M_⊙), the stellar tracks predicted by the two codes become increasingly divergent with increasing mass and wind mass-loss efficiency. Notably, the maximal radial extent, which is crucial for determining whether the components of the triple systems interact, can differ by up to two orders of magnitude between the two stellar codes in the considered mass range and with a standard mass-loss efficiency. We demonstrate that this has significant implications for the predicted evolutionary pathways of triple systems and leads to divergences between the predictions of simulations performed with MESA and SEBA. In particular, we show that using MESA as the stellar code, the minimum period for avoiding inner mass transfer is reduced by three orders of magnitude compared to what is predicted by SEBA. We highlight that this result has consequences for the formation of gravitational wave sources through the triple compact object channel.

Conclusions. Our simulations offer new insights into the physics of triple star systems, as some relevant processes (mass-loss by stellar winds, radial expansion, tides, precession due to the distortion of the stars) can be treated more self-consistently. They indicate that the results of triple system population synthesis studies must be interpreted cautiously, in particular when the considered masses are outside the range of the grid the fast codes are based on and when significant stellar winds are considered.