Circumbinary Accretion from Finite and Infinite Disks

Abstract

We carry out 2D viscous hydrodynamics simulations of circumbinary disk (CBD) accretion using AREPO. We resolve the accretion flow from a large-scale CBD down to the streamers and disks around individual binary components. Extending our recent studies (Muñoz et al. 2019), we consider circular binaries with various mass ratios (0.1 ≤ q b ≤ 1) and study accretion from "infinite", steady-supply disks and from finite-sized, viscously spreading tori. For "infinite" disks, a global steady state can be reached, and the accretion variability has a dominant frequency ∼0.2Ω b for q b > 0.5 and Ω b for q b < 0.5, (Ω b is the binary angular frequency). We find that the accretion "eigenvalue" l 0 -the net angular momentum transfer from the disk to the binary per unit accreted mass -is always positive and falls in the range (0.65-0.85)a 2 b Ω b (with a b the binary separation), depending weakly on the mass ratio and viscosity. This leads to binary expansion when q b 0.3. Accretion from a finite torus can be separated into two phases: an initial transient phase, corresponding to the filling of the binary cavity, followed by a viscous pseudo-stationary phase, during which the torus viscously spreads and accretes onto the binary. In the viscous phase, the net torque on the binary per unit accreted mass is close to l 0 , the value derived for "infinite" disks. We conclude that similar-mass binaries accreting from CBDs gain angular momentum and expand over long time scales. This result significantly impacts the coalescence of supermassive binary black holes and newly formed binary stars. We offer a word of caution against conclusions drawn from simulations of transient accretion onto empty circumbinary cavities.

Disc fragmentation and intermittent accretion on to supermassive stars

“…Its orbital distance from the central star is 2000 au in the early phase (∼ 5 kyr) and gradually increases with time, finally reaching 9000 (filamentary) and 4000 au (spherical), respectively. We expect the separation will increase even after that owing to the acquisition of angular momentum by the gas accretion, as suggested in recent simulations of binary accretion (Duffell et al 2020;Muñoz et al 2020). In fact, long-term simulations in Latif et al (2020) demonstrated the formation of binary SMSs with a wide separation (∼pc).…”

Periodic Eruptive Variability of the Isolated Pre-main-sequence Star V347 Aurigae

“…The suggestion is that high eccentricity is required in order to bring the components close enough together, perhaps within only a few R star of one another, in order to periodically influence the disk-to-star accretion flow. Circumbinary accretion scenarios as a function of eccentricity and mass ratio have been studied in detail recently by, e.g., Muñoz et al (2020) and Duffell et al (2020). We highlight that all of the exemplar pulsed accretion binaries have shorter periods and lower amplitude variability than the case of V347 Aur, and that the binary simulations also predict maximum accretion rate variations that do not exceed 50%-100%.…”

Binary black hole mergers in AGN accretion discs: gravitational wave rate density estimates

“…In contrast to the above conclusions, recent 2D and 3D viscous hydrodynamical simulations of circumbinary accretion indicate that accreting binaries consistently gain angular momentum from the disc (Moody et al 2019;Muñoz et al 2019Muñoz et al , 2020. This implies that the binaries expand as they accrete, with positive ȧ > 0 (for moderate mass ratios).…”