How binaries accrete: hydrodynamics 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, r a, 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.

How cooling influences circumbinary discs

“…However, this does not affect the structure of the disc in a significant way. We investigated this by lowering the inner radius in locally isothermal test runs and found that 1 a bin offers accurate results while maintaining a reasonable timestep, in agreement with Tiede et al (2021) who showed that material within inside 1 a bin is accreted and does not change the dynamics of the outer disc.…”

“…In this study we chose an inner domain boundary at 1 a bin such that the full interaction of the binary is not included for runtime reasons. Tiede et al (2021) shows that inside this radius nearly all mass is accreted and does not alter the dynamics of the outer disc. As Fig.…”

“…The effect of the disc on a binary object was recently studied by Muñoz et al (2019 and Tiede et al (2020), discovering binary expansion for thick, viscous discs (α ≥ 0.01, h = 0.1). Tiede et al (2021) looked into the accretion dynamics with tracer particles embedded in a Cartesian grid simulation. The effects of the binary eccentricity on the momentum exchange between the binary components was investigated in models by D 'Orazio & Duffell (2021).…”

On the origin of the lump in circumbinary discs

“…In the attempt to understand the final stages prior to merger of binary black holes (BBHs), numerical studies have addressed the problem of circumbinary accretion flows, taking the form of a disc, around a central BBH. Thanks to these studies, a global picture of the accretion structures under such circumstances has emerged, including a cavity and two streams feeding the individual BHs (MacFadyen & Milosavljević 2008, Shi et al 2012, Noble et al 2012, D'Orazio et al 2013, Gold et al 2014, Shi & Krolik 2015, Ragusa et al 2016, Armengol et al 2021, Tiede et al 2021, Liu 2021). In addition to those, the most surprising feature is certainly an overdensity, localized radially and azimuthally, which is found to orbit at the Keplerian period at the circumbinary disc inner edge, while being swept off by the spiral waves associated to the binary and therefore to the binary orbital period.…”

A Survey of Disc Thickness and Viscosity in Circumbinary Accretion: Binary Evolution, Variability, and Disc Morphology

“…Near the circumbinary disc periapse, material can be captured by the binary or flung away from it through the cavity (e.g. Tiede et al 2021). Additionally, accretion discs ('minidiscs') form around each binary component.…”