Circumbinary Accretion from Finite and Infinite Disks. (arXiv:1910.04763v1 [astro-ph.HE])

<a href="http://arxiv.org/find/astro-ph/1/au:+Munoz_D/0/1/0/all/0/1">Diego Muñoz</a> (CIERA), <a href="http://arxiv.org/find/astro-ph/1/au:+Lai_D/0/1/0/all/0/1">Dong Lai</a> (Cornell), <a href="http://arxiv.org/find/astro-ph/1/au:+Kratter_K/0/1/0/all/0/1">Kaitlin Kratter</a> (Arizona), <a href="http://arxiv.org/find/astro-ph/1/au:+Miranda_R/0/1/0/all/0/1">Ryan Miranda</a> (IAS)

We carry out 2D viscous hydrodynamics simulations of circumbinary accretion

using the moving-mesh code AREPO. Our simulations resolve the accretion flow

over a wide range of scales, from the large circumbinary disk (CBD) to

accretion streams and disks around individual binary components. Extending our

recent studies (Mu~noz et al 2019), we consider circular binaries with various

mass ratios ($0.4leq q_bleq 1$) and explore accretion from “infinite”,

steady-supply disks and from finite-sized, viscously spreading tori. In the

“infinite” disk case, a global steady state can be reached, and we find that

the accretion “eigenvalue” $l_0$, defined as the net angular momentum transfer

from the disk to the binary per unit accreted mass, is always positive and

falls in the range ($0.65$-$0.75)a_{rm b}^2Omega_{rm b}$ (where $a_{rm b}$,

$Omega_{rm b}$ are the binary separation and angular frequency), depending

weakly on the mass ratio and disk viscosity. This positive $l_0$ leads to

expansion of the binary separation. Binary 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. We find

that, in the stationary viscous phase, the net torque on the binary per unit

accreted mass is close to the accretion “eigenvalue” $l_0$ derived for the

steady-supply disks. We conclude that in general, binaries accreting from

circumbinary disks/tori gain angular momentum and expand over long time scales.

This result can significantly impact the evolution and coalescence of

supermassive binary black holes and newly formed binary stars. We offer a word

of caution when drawing conclusions from simulations of transient accretion

onto empty circumbinary cavities.

We carry out 2D viscous hydrodynamics simulations of circumbinary accretion

using the moving-mesh code AREPO. Our simulations resolve the accretion flow

over a wide range of scales, from the large circumbinary disk (CBD) to

accretion streams and disks around individual binary components. Extending our

recent studies (Mu~noz et al 2019), we consider circular binaries with various

mass ratios ($0.4leq q_bleq 1$) and explore accretion from “infinite”,

steady-supply disks and from finite-sized, viscously spreading tori. In the

“infinite” disk case, a global steady state can be reached, and we find that

the accretion “eigenvalue” $l_0$, defined as the net angular momentum transfer

from the disk to the binary per unit accreted mass, is always positive and

falls in the range ($0.65$-$0.75)a_{rm b}^2Omega_{rm b}$ (where $a_{rm b}$,

$Omega_{rm b}$ are the binary separation and angular frequency), depending

weakly on the mass ratio and disk viscosity. This positive $l_0$ leads to

expansion of the binary separation. Binary 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. We find

that, in the stationary viscous phase, the net torque on the binary per unit

accreted mass is close to the accretion “eigenvalue” $l_0$ derived for the

steady-supply disks. We conclude that in general, binaries accreting from

circumbinary disks/tori gain angular momentum and expand over long time scales.

This result can significantly impact the evolution and coalescence of

supermassive binary black holes and newly formed binary stars. We offer a word

of caution when drawing conclusions from simulations of transient accretion

onto empty circumbinary cavities.

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