Circumbinary Accretion: From Binary Stars to Massive Binary Black Holes

Abstract

We review recent works on the dynamics of circumbinary accretion, including time variability, angular momentum transfer between the disk and the binary, and the secular evolution of accreting binaries. These dynamics impact stellar binary formation/evolution, circumbinary planet formation/migration, and the evolution of (super)massive black hole binaries. We discuss the dynamics and evolution of inclined/warped circumbinary disks and connect with observations of protoplanetary disks. A special kind of circumbinary accretion involves binaries embedded in big disks, which may contribute to the mergers of stellar-mass black holes in AGN disks. Highlights include the following: ▪ Circumbinary accretion is highly variable, being modulated at Pb (the binary period) or ∼5 Pb, depending on the binary eccentricity eb and mass ratio qb. ▪ The inner region of the circumbinary disk can develop coherent eccentric structure, which may modulate the accretion and affect the physical processes (e.g., planet migration) taking place in the disk. ▪ Over long timescales, circumbinary accretion steers binaries toward equal masses, and it does not always lead to binary orbital decay. The secular orbital evolution depends on the binary parameters ( eb and qb) and on the thermodynamic properties of the accreting gas. ▪ A misaligned disk around a low-eccentricity binary tends to evolve toward coplanarity due to viscous dissipation. But when eb is significant, the disk can evolve toward “polar alignment,” with the disk plane perpendicular to the binary plane. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 61 is August 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Observational Evidence of a Centi-parsec Supermassive Black Hole Binary Existing in the Nearby Galaxy M81

“…In the case of mass ratios =1, the accretion and variability are dominated by the accretion flow onto the primary (Gold et al 2014). As proposed in Section 4, the jet precession of the primary black hole is due to the misaligned disk to the orbital plane (Katz et al 1982;Bate et al 2000;Lai & Muñoz 2023). We cannot distinguish the scenario of disk-jet precession or precession of the disk jet and its central black hole as a whole either.…”

“…While the inspiral from 0.02-0.004 pc could be driven by the gas-assisted orbital contraction, which depends on the accretion dynamics of the circumbinary disk and the binary system itself. This is related to the interesting final parsec problem (Begelman et al 1980;Lai & Muñoz 2023) and should be investigated with further studies.…”

Long-term Evolution of Massive-star Post-common-envelope Circumbinary Disks and the Environments of Fast Luminous Transients

“…Tidal interactions exchange angular momentum between the gas disk and binary (Lin & Papaloizou 1979;Goldreich & Tremaine 1980;Pringle 1991). The binary mass ratio is sufficiently large (M • /M c ∼ 0.1-1) to open a gap in the disk (e.g., Artymowicz & Lubow 1994;Lai & Muñoz 2023). A growing body of work explores the magnitude and sign of the torque, and how these vary with the disk properties (e.g., Duffell et al 2020;Muñoz et al 2020;Gagnier & Pejcha 2023;Siwek et al 2023; see Section 3.2).…”

“…The interplay between these effects is complex, with ongoing debate regarding how the net torque depends on the disk and binary properties. The binary orbital evolution is conveniently characterized by the accretion eigenvalue (Lai & Muñoz 2023),…”

Detection of a ∼0.1c Radio Knot in M81* Associated with a Moderate X-Ray Flare

“…The discrete knot could be associated with ongoing circumbinary accretion around a massive black hole binary. For an equal-mass-ratio massive binary in an eccentric orbit, accretion onto the primary and secondary component of the binary will experience symmetry breaking, with the secondary black hole accreting about one order of magnitude more material than the other; this accretion pattern reverses over the long-term precession timescale (e.g., see the reviews by Gold 2019;Lai & Muñoz 2023). For unequal-mass eccentric binaries, it is also possible that the secondary black hole accretes much less mass than the primary when the circumbinary disk undergoes forced precession, which happens for an even larger orbital eccentricity (Gold et al 2014;Siwek et al 2023).…”