TY - JOUR

T1 - Hydrodynamics of Circumbinary Accretion

T2 - Angular Momentum Transfer and Binary Orbital Evolution

AU - Munoz, Diego J.

AU - Miranda, Ryan

AU - Lai, Dong

N1 - Funding Information:
D.J.M. thanks Volker Springel for making AREPO available for use in this work. We thank Kaitlin Kratter, Yoram Lithwick, Zsolt Regály, Pawel Artymowicz, and Zoltán Haiman for useful discussion and Luke Zoltan Kelley for comments on the manuscript. This work is supported in part by the NSF grant AST1715246 and NASA grant NNX14AP31G. This research was supported in part through the computational resources and staff contributions provided for the Quest high-performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology.
Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved.

PY - 2019/1/20

Y1 - 2019/1/20

N2 - We carry out 2D viscous hydrodynamical simulations of circumbinary accretion using the moving-mesh code AREPO. We self-consistently compute the accretion flow over a wide range of spatial scales, from the circumbinary disk (CBD) far from the central binary, through accretion streamers, to the disks around individual binary components, resolving the flow down to 2% of the binary separation. We focus on equal-mass binaries with arbitrary eccentricities. We evolve the flow over long (viscous) timescales until a quasi-steady state is reached, in which the mass supply rate at large distances Ṁ 0 (assumed constant) equals the time-averaged mass transfer rate across the disk and the total mass accretion rate onto the binary components. This quasi-steady state allows us to compute the secular angular momentum transfer rate onto the binary, {J}, and the resulting orbital evolution. Through direct computation of the gravitational and accretional torques on the binary, we find that {J} is consistently positive (i.e., the binary gains angular momentum), with l0 {J} Ṁ 0 in the range of (0.4 - 0.8)a b 2 ω b, depending on the binary eccentricity (where ȧb, ωb are the binary semimajor axis and angular frequency); we also find that this {J} is equal to the net angular momentum current across the CBD, indicating that global angular momentum balance is achieved in our simulations. In addition, we compute the timeaveraged rate of change of the binary orbital energy for eccentric binaries and thus obtain the secular rates{ȧb} and {ėb} . In all cases, {ȧb} is positive; that is, the binary expands while accreting. We discuss the implications of our results for the merger of supermassive binary black holes and for the formation of close stellar binaries.

AB - We carry out 2D viscous hydrodynamical simulations of circumbinary accretion using the moving-mesh code AREPO. We self-consistently compute the accretion flow over a wide range of spatial scales, from the circumbinary disk (CBD) far from the central binary, through accretion streamers, to the disks around individual binary components, resolving the flow down to 2% of the binary separation. We focus on equal-mass binaries with arbitrary eccentricities. We evolve the flow over long (viscous) timescales until a quasi-steady state is reached, in which the mass supply rate at large distances Ṁ 0 (assumed constant) equals the time-averaged mass transfer rate across the disk and the total mass accretion rate onto the binary components. This quasi-steady state allows us to compute the secular angular momentum transfer rate onto the binary, {J}, and the resulting orbital evolution. Through direct computation of the gravitational and accretional torques on the binary, we find that {J} is consistently positive (i.e., the binary gains angular momentum), with l0 {J} Ṁ 0 in the range of (0.4 - 0.8)a b 2 ω b, depending on the binary eccentricity (where ȧb, ωb are the binary semimajor axis and angular frequency); we also find that this {J} is equal to the net angular momentum current across the CBD, indicating that global angular momentum balance is achieved in our simulations. In addition, we compute the timeaveraged rate of change of the binary orbital energy for eccentric binaries and thus obtain the secular rates{ȧb} and {ėb} . In all cases, {ȧb} is positive; that is, the binary expands while accreting. We discuss the implications of our results for the merger of supermassive binary black holes and for the formation of close stellar binaries.

KW - accretion

KW - accretion disks

KW - binaries: general

KW - black hole physics

KW - stars: pre-main sequence

UR - http://www.scopus.com/inward/record.url?scp=85062040334&partnerID=8YFLogxK

U2 - 10.3847/1538-4357/aaf867

DO - 10.3847/1538-4357/aaf867

M3 - Article

AN - SCOPUS:85062040334

SN - 0004-637X

VL - 871

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 1

M1 - 84

ER -