Revisiting the picture of circumbinary disc truncation

Context. Discs surrounding binaries, referred to as circumbinary discs, are widely observed to develop central cavities that are carved by the gravitational influence of the binary. Aims. Analytical estimates of cavity sizes predict truncation at ~2–3 binary separations, depending on binary properties. However, numerical studies show only qualitative agreement with these predictions, as cavity sizes often evolve on long timescales and can substantially exceed the analytically predicted values. In this work, we revise this paradigm, suggesting that tidal truncation in circumbinary discs responds to additional dynamical parameters that have so far been neglected. Methods. We analysed a suite of 80 numerical simulations of circumbinary discs to re-examine the physical mechanism responsible for cavity truncation and to provide a prescription for the cavity size independent of the state of evolution of the system. Results. We find that truncation depends not only on the binary parameters abin, ebin and mass ratio q but also on the instantaneous cavity eccentricity ecav and the relative apsidal orientation ϖbin – ϖcav. These quantities jointly determine the pericentre of the innermost stable disc orbit, Rp, in a way that shares some similarities with orbital stability in the restricted three-body problem. Hydrodynamical effects introduce secondary corrections, with the disc scale height H and viscosity α mildly shifting the cavity edge relative to the purely gravitational prediction. We introduce a semi-analytical prescription that captures these dependences for Rp and the cavity semi-major axis acav. Additional variability arises from resonant oscillations of the cavity, characterised by anti-phase evolution of ecav and acav, which are not captured by the model. Conclusions. We conclude that cavity truncation for binaries with mass ratios q > 0.05 is a process where the instantaneous orbital properties of the disc (ecav, ϖcav) play a fundamental role and should be taken into account to accurately evaluate the truncation efficiency.