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

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 Mu&#xf1;oz</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.

http://arxiv.org/icons/sfx.gif

Comments are closed.