Silvia Toonen

Barium stars have been studied extensively over the past few decades, yet our current understanding of how these intriguing objects formed leaves much to be desired. Many trends observed in systems containing barium stars cannot be satisfactorily explained by classical binary evolution models, naturally raising the question of whether triples and other higher order multiples can give rise to such exotic objects. In this paper, we study the possibility that a Roche Lobe overflow from a tertiary in a hierarchical triple system can potentially lead to surface barium enrichment within the inner binary, while at the same time causing the inner binary to merge, thereby producing a barium star. This possibility has the potential to form a large proportion of Barium stars, as Roche Lobe overflow from a tertiary is typically much more stable for close orbits than that from a binary companion. Various formation channels and mechanisms by which this can be achieved are considered, and constraints on relative formation rates are placed on each scenario. Three recently discovered triple systems containing Ba stars further support our proposed formation mechanism.

T Pyxidis is one of the most studied recurrent novae and even the prototype of its class. However, its mass transfer rate is three to four orders of magnitude above expected values based on GW radiation. Here, we show that T Pyx has a distant proper motion companion and therefore likely evolved from a hierarchical triple star system. Triple evolution can naturally produce exotic compact binaries as a result of three-body dynamics, either by Kozai-Lidov eccentricity cycles in dynamically stable systems or via mass-loss-induced dynamical instabilities. By numerically evolving triple progenitors with physically reasonable parameters forward in time, we show explicitly that the inner binary can become so eccentric that mass transfer is triggered at periastron, driving the secondary out of thermal equilibrium. We suggest that short-period recurrent novae likely evolved via this extreme state, explaining their departure from standard binary evolution tracks.

Destabilising triples are ideal laboratories for studying the interplay between orbital dynamics and stellar evolution, however our current understanding of the evolution of unstable triple systems is mainly built upon results from extensive binary-single scattering experiments. Contrary to commonly expected, the majority of triples (54-69%) preserve their hierarchy throughout their evolution, in stead of a chaotic, democratic resonant interaction. The duration of the unstable phase is much longer than expected (1000 − 10000 crossing times, reaching up to millions), so that long-term stellar evolution effects cannot be neglected. Furthermore, collisions are common, but between MS stars instead of giant stars. A promising indicator for distinguishing triples that will follow the democratic or hierarchical route, is the relative inclination between the inner and outer orbits. Its influence can be summed up in two rules of thumb: (1) prograde triples tend to evolve towards hierarchical collisions and ejections, and (2) retrograde triples tend to evolve towards democratic encounters and a loss of initial hierarchy, unless the system is compact, which experience collision preferentially.