Return of the TEDI: revisiting the Triple Evolution Dynamical Instability channel in triple stars

TL;DR

This study revisits the Triple Evolution Dynamical Instability (TEDI) in triple star systems using advanced population synthesis, confirming its significance in stellar collisions and exploring its broader astrophysical consequences.

Contribution

It employs state-of-the-art models to self-consistently analyze TEDI, including stellar evolution, binary interactions, and gravitational perturbations, providing updated collision rate estimates and insights into resulting stellar phenomena.

Findings

Galactic TEDI collision rate ~1e-4/yr, consistent with previous estimates.

Majority of collisions involve main sequence stars, potentially forming blue stragglers.

TEDI also produces unbound stars at a rate of ~1e-4/yr, with no high-velocity escapers.

Abstract

Triple-star systems exhibit a phenomenon known as the Triple Evolution Dynamical Instability (TEDI), in which mass loss in evolving triples triggers short-term dynamical instabilities, potentially leading to collisions of stars, exchanges, and ejections. Previous work has shown that the TEDI is an important pathway to head-on stellar collisions in the Galaxy, significantly exceeding the rate of collisions due to random encounters in globular clusters. Here, we revisit the TEDI evolutionary pathway using state-of-the-art population synthesis methods that self-consistently take into account stellar evolution and binary interactions, as well as gravitational dynamics and perturbations from passing stars in the field. We find Galactic TEDI-induced collision rates on the order of 1e-4/yr, consistent with previous studies which were based on more simplified methods. The majority of TEDI-induced collisions involve main sequence stars, potentially producing blue straggler stars. Collisions are also possible involving more evolved stars, potentially producing eccentric post-common-envelope systems, and white dwarfs collisions leading to Type Ia supernovae (although the latter with a negligible contribution to the Galactic rate). In our simulations, the TEDI is not only triggered by adiabatic wind mass loss, but also by Roche lobe overflow in the inner binary: when the donor star becomes less massive than the accretor, the inner binary orbit widens, triggering triple dynamical instability. We find that collision rates are increased by ~17% when fly-bys in the field are taken into account. In addition to collisions, we find that the TEDI produces ~1e-4/yr of unbound stars, although none with escape speeds in excess of 1e3 km/s.