2022ApJ...925..178H

Triple-star systems exhibit a phenomenon known as 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 10^–4 yr^–1, 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 involving more evolved stars are also possible, 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 flybys in the field are taken into account. In addition to collisions, we find that the TEDI produces ∼10^–4 yr^–1 of unbound stars, although none with escape speeds in excess of 10³ km s^–1.