Implementation of Sink Particles in the Athena Code
  We describe implementation and tests of sink particle algorithms in the
Eulerian grid-based code Athena. Introduction of sink particles enables
long-term evolution of systems in which localized collapse occurs, and it is
impractical (or unnecessary) to resolve the accretion shocks at the centers of
collapsing regions. We discuss similarities and differences of our methods
compared to other implementations of sink particles. Our criteria for sink
creation are motivated by the properties of the Larson-Penston collapse
solution. We use standard particle-mesh methods to compute particle and gas
gravity together. Accretion of mass and momenta onto sinks is computed using
fluxes returned by the Riemann solver. A series of tests based on previous
analytic and numerical collapse solutions is used to validate our method and
implementation. We demonstrate use of our code for applications with a
simulation of planar converging supersonic turbulent flow, in which multiple
cores form and collapse to create sinks; these sinks continue to interact and
accrete from their surroundings over several Myr.
