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

TL;DR

This study uses long-term 2D hydrodynamic simulations to explore how viscous circumbinary protoplanetary discs influence the orbital evolution of binary stars, revealing complex interactions involving disc precession, eccentricity, and mass exchange.

Contribution

It provides new insights into the long-term dynamical interactions between viscous circumbinary discs and binary orbits, emphasizing the role of disc precession and viscosity in orbital evolution.

Findings

Disc precession significantly affects binary eccentricity.

Thin discs cause binary orbit shrinking even at low viscosity.

Disc interactions can excite binary eccentricity in a range of initial eccentricities.

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 like viscosity and scale height, and the disc and cavity shape influences 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 ($H/R<0.05$) 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 circularisation or excitation are nearly balanced, and the evolution of the binary semi-major axis and eccentricity will be sensitive to the ratio of mass accretion between the secondary and primary components.