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We implemented sink particles in the adaptive mesh refinement (AMR)hydrodynamics code FLASH. Sink particles are created in regions of localgravitational collapse, and their trajectories and accretion can be followedover many dynamical times. We perform a series of tests including the timeintegration of circular and elliptical orbits, the collapse of a Bonnor-Ebertsphere and a rotating, fragmenting cloud core. We compare the collapse of ahighly unstable singular isothermal sphere to the theory by Shu (1977), andshow that the sink particle accretion rate is in excellent agreement with thetheoretical prediction.  To model eccentric orbits and close encounters of sink particles accurately,we show that a very small timestep is often required, for which we implementedsubcycling of the N-body system. We emphasize that a sole density threshold forsink particle creation is insufficient in supersonic flows, if the densitythreshold is below the opacity limit. In that case, the density can exceed thethreshold in strong shocks that do not necessarily lead to local collapse.Additional checks for bound state, gravitational potential minimum, Jeansinstability and converging flows are absolutely necessary for a meaningfulcreation of sink particles.  We apply our new sink particle module for FLASH to the formation of a stellarcluster, and compare to a smoothed particle hydrodynamics (SPH) code with sinkparticles. Our comparison shows encouraging agreement of gas properties,indicated by column density distributions and radial profiles, and of sinkparticle formation times and positions. We find excellent agreement in thenumber of sink particles formed, and in their accretion and mass distributions.

Modeling Collapse and Accretion in Turbulent Gas Clouds: Implementation and Comparison of Sink Particles in AMR and SPH