Circumbinary discs: Numerical and physical behaviour

Aims. Discs around a central binary system play an important role in star and planet formation and in the evolution of galactic discs. These circumbinary discs are strongly disturbed by the time varying potential of the binary system and display a complex dynamical evolution that is not well understood. Our goal is to investigate the impact of disc and binary parameters on the dynamical aspects of the disc.

Methods. We study the evolution of circumbinary discs under the gravitational influence of the binary using two-dimensional hydrodynamical simulations. To distinguish between physical and numerical effects we apply three hydrodynamical codes. First we analyse in detail numerical issues concerning the conditions at the boundaries and grid resolution. We then perform a series of simulations with different binary parameters (eccentricity, mass ratio) and disc parameters (viscosity, aspect ratio) starting from a reference model with Kepler-16 parameters.

Results: Concerning the numerical aspects we find that the length of the inner grid radius and the binary semi-major axis must be comparable, with free outflow conditions applied such that mass can flow onto the central binary. A closed inner boundary leads to unstable evolutions. We find that the inner disc turns eccentric and precesses for all investigated physical parameters. The precession rate is slow with periods ($T_{prec}$) starting at around $500$ binary orbits ($T_{bin}$) for high viscosity and a high aspect ratio $H/R$ where the inner hole is smaller and more circular. Reducing $\alpha$ and $H/R$ increases the gap size and $T_{prec}$ reaches $2500 \ T_{bin}$. For varying binary mass ratios $q_{bin}$ the gap size remains constant, whereas $T_{prec}$ decreases with increasing $q_{bin}$. For varying binary eccentricities $e_bin$ we find two separate branches in the gap size and eccentricity diagram. The bifurcation occurs at around $e_{crit} \approx 0.18$ where the gap is smallest with the shortest $T_{prec}$. For $e_{bin}$ lower and higher than $e_{crit}$, the gap size and $T_{prec}$ increase. Circular binaries create the most eccentric discs.

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Recommended citation: Thun, Kley & Picogna. (2017). “Circumbinary discs: Numerical and physical behaviour.” Astronomy & Astrophysics. Volume 604, id.A102, 19 pp.