Binary orbital evolution driven by a circumbinary disc

Understand

The question whether the interaction of a circumbinary disc with the central binary system leads to shrinking or expansion of the binary orbit has attracted considerable interest as it impacts the evolution of binary black holes and stellar binary stars in their formation phase.

• We performed two-dimensional hydrodynamical simulations of circumbinary discs for a large parameter set of disc viscosities and thicknesses and two different binary mass ratios for binaries on circular orbits.
• For those we measured carefully the net angular momentum and mass transfer between disc and binary system, and evaluate the normalised specific angular momentum accretion, $j_\mathrm{s}$ .
• This is compared to the theoretical, critical specific angular momentum change $j_\mathrm{s,crit}$ that separates contracting from expanding cases which depends on the the binary's mass ratio and the relative accretion onto the two stars.