In recent years the detection of a large variety of structures in protoplanetary systems (once believed to be featureless), the detection of a high number of exoplanets with properties much different from those detected in the solar system, and the detection of gravitational waves from binary black holes have all raised a similar fundamental research question: “How does the mutual interaction between a binary and its surrounding gas shape the systems we observe?”
The conservation of angular momentum during the infall of material on to a binary (which in this context can mean two stars, two black holes, or a star and a planet) forces the material to form a disc structure that orbits the binary, and the dynamics of these so-called circumbinary discs have been studied for many years. The binary exerts a so-called “tidal torque” on the accretion disc, carving a gap or even a cavity in the disc. The disc in turn exerts a back-reaction torque on the binary, changing its orbit, causing it to migrate and/or changing its eccentricity. The traditional theoretical approach requires some simplifying assumptions, in order to make the equations analytically tractable, such as assuming that the mass of the secondary, or the mass of the disc, is much smaller than the primary. When these assumptions are not satisfied the theoretical predictions become unreliable, and we must turn to numerical hydrodynamic simulations in order to investigate the problem properly. However, the time-scales involved in this problem are long (at least hundreds of binary orbits), and high numerical resolution is required to resolve the tidal torques correctly. As a result, accurate hydrodynamical simulations of binary-disc interactions are extremely computationally challenging.
By exploiting the power of DiRAC, we were able to perform a suite of 3-D Smoothed Particle Hydrodynamics simulations which follow the evolution of a wide range of different circumbinary discs for long timescales (typically 2000 binary orbits). Our main aim was to understand the physical mechanism responsible for the formation of the so called “horseshoe” structures that have been seen in a large number of recent simulations of circumbinary discs, and which have been proposed as an explanation for the asymmetric structures observed by ALMA in numerous planet-forming systems. Our simulations (shown in Fig.1) showed that horseshoe structures are associated with the rapid growth of the cavity size when the disc becomes eccentric, if the binary companion is sufficiently massive. The long-term evolution of our simulations suggests that these features are actually transient, but they can survive for thousands of binary orbits. In real systems this corresponds to a significant fraction (~10%) of the ~Myr lifetimes of protoplanetary discs, which may explain why these structures are only seen in a sub-set of observations. Our simulations have provided important new insight into an important theoretical problem, as well as raising some interesting new questions. In particular, we unexpectedly discovered a very strong relationship between the growth of the disc cavity and its eccentricity, and our future work with DiRAC will investigate the physics behind this result in much greater detail.