How Binaries Accrete: Hydrodynamic Simulations with Passive Tracer Particles

Abstract

Linear analysis of gas flows around orbiting binaries suggests that a centrifugal barrier ought to clear a low-density cavity around the binary and inhibit mass transfer onto it. Modern hydrodynamics simulations have confirmed the low-density cavity, but show that any mass flowing from large scales into the circumbinary disk is eventually transferred onto the binary components. Even though many numerical studies confirm this picture, it is still not understood precisely how gas parcels overcome the centrifugal barrier and ultimately accrete. We present a detailed analysis of the binary accretion process, using an accurate prescription for evolving grid-based hydrodynamics with Lagrangian tracer particles that track the trajectories of individual gas parcels. We find that binary accretion can be described in four phases: (1) gas is viscously transported through the circumbinary disk up to the centrifugal barrier at the cavity wall, (2) the cavity wall is tidally distorted into accretion streams consisting of near-ballistic gas parcels on eccentric orbits, (3) the portion of each stream moving inwards of an ``accretion horizon'' radius $\bar{r}\simeq a$ -- the radius beyond which no material is returned to the cavity wall -- becomes bound to a minidisk orbiting an individual binary component, and (4) the minidisk gas accretes onto the binary component through the combined effect of viscous and tidal stresses.

Published in ApJ; 18 pages, 16 figures

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• On the Role of Gas Cooling in the Dynamics of Circumbinary Disks
• Eccentric Minidisks in Accreting Binaries
• Viscous circumbinary protoplanetary discs -- I. Structure of the inner cavity
• Suppressed accretion onto massive black hole binaries surrounded by thin disks
• On the origin of the lump in circumbinary discs
• Radiation Magnetohydrodynamic Simulation of sub-Eddington Circumbinary Disk around an Equal-mass Massive Black Hole Binary
• Hot, cold, and multi-component accretion flows around supermassive black hole binaries
• Analytical and Numerical Analysis of Circumbinary Disk Dynamics -- I: Coplanar Systems
• Stellar Stripping and Disruption in Disks around Supermassive Black Hole Binaries: Repeating nuclear transients prior to LISA events
• Measuring the Numerical Viscosity in Simulations of Protoplanetary Disks in Cartesian Grids -- The Viscously Spreading Ring Revisited
• Electromagnetic variability from circumbinary discs around binary black holes during their post-decoupling epoch
• Viscous circumbinary protoplanetary discs -- II. Disc effects on the binary orbit