Viscous circumbinary protoplanetary discs – II. Disc effects on the binary orbit

ABSTRACT More than half of all stars are part of binaries, and many form in a common circumbinary disc. The interaction with the binary shapes the disc to feature a large eccentric inner cavity and spirals in the inner disc. The shape of the cavities is linked to binary and disc properties such as viscosity and scale height, and the disc and cavity shape influence the orbital evolution of the binary stars. This is the second part of the study in which we use 2D hydrodynamic long-term simulations for a range of viscous parameters relevant to protoplanetary discs to understand the interaction between young stars and the circumbinary disc. The long-term simulations allow us to study how disc shape and exchange of mass, momentum, and energy between binary and disc depend on the precession angle between disc and binary orbit on time-scales of thousands of binary orbits. We find a considerable, periodic interaction between the precession of the disc and the binary eccentricity that can significantly exceed the precession-averaged change in eccentricity. We further confirm that thin discs ($H/R\lt 0.05$) lead to shrinking binary orbits, also in the regime of low viscosity, $\alpha =10^{-3}$. In general, the disc can excite eccentricity in binaries with initial eccentricities in the range of $e_\mathrm{bin}=0.05\!\!-\!\!0.4$. In most cases, the terms aiding shrinking or expansion and circularization or excitation are nearly balanced, and the evolution of the binary semimajor axis and eccentricity will be sensitive to the ratio of mass accretion between the secondary and primary components.