How binaries accrete: hydrodynamics simulations with passive tracer
  particles
  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, $\bar r \simeq a$, 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.
