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

Abstract

Star formation is such a complex process that accurate numerical tools are needed to quantitatively examine the mass distribution and accretion of fragments in collapsing, turbulent, magnetized gas clouds. To enable a numerical treatment of this regime, we implemented sink particles in the adaptive mesh refinement (amr) hydrodynamics code flash. Sink particles are created in regions of local gravitational collapse, and their trajectories and accretion can be followed over many dynamical times. We perform a series of tests including the time integration of circular and elliptical orbits, the collapse of a Bonnor-Ebert sphere and a rotating, fragmenting cloud core. We compare the collapse of a highly unstable singular isothermal sphere to the theory by Shu (1977), and show that the sink particle accretion rate is in excellent agreement with the theoretical 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 implemented subcycling of the N -body system. We emphasize that a sole density threshold for sink particle creation is insufficient in supersonic flows, if the density threshold is below the opacity limit. In that case, the density can exceed the threshold in strong shocks that do not necessarily lead to local collapse. Additional checks for bound state, gravitational potential minimum, Jeans instability and converging flows are absolutely necessary for a meaningful creation of sink particles.We apply our new sink particle module for flash to the formation of a stellar cluster, and compare to a smoothed particle hydrodynamics (sph) code with sink particles. Our comparison shows encouraging agreement of gas properties, indicated by column density distributions and radial profiles, and of sink particle formation times and positions. We find excellent agreement in the number of sink particles formed, and in their accretion and mass distributions.

Accretion and magnetic field morphology around Class 0 stage protostellar discs

“…We make use of the sink particle routine (Federrath et al 2010) to model the formation of protostars. The results of the simulations were first presented in Seifried et al (2012Seifried et al ( , 2013.…”

“…We introduce sink particles above a density threshold of ρcrit = 1.14·10 −10 g cm −3 using an accretion radius of 3.14 AU (see Federrath et al 2010, for details). Here we only point out that the angular momentum carried by the gas will be transferred to the spin of the sink particle.…”

Assessing inflow rates in atomic cooling haloes: implications for direct collapse black holes

“…The general criteria used to create sink particles include assuming a density threshold, requiring the highest level of refinement, having converging flow, or considering the local minimum of gravitational potential and/or a gravitationally bound region. In the literature, various combinations of these approaches are used and a detailed comparison of these methods was made by Federrath et al (2010). They found that a simple density threshold criterion to create sink particles is insufficient in the regime of supersonic turbulence because of local density enhancements.…”

“…Sink velocities are computed based on momentum conservation. The works of Federrath et al (2010) and Bleuler & Teyssier (2014) suggest that this method of sink creation may create more sink particles. However, this may happen only in the regime where efficient fragmentation is expected, in the presence of supersonic turbulence or efficient cooling.…”

The Formation of Massive Stars

“…Numerical simulations show that molecular clouds fragment down to their thermal or magnetic Jeans masses (Bonnell et al 1991;Klessen et al 1998;Klessen 2001;Bate et al 2003;Jappsen et al 2005;Federrath et al 2010). As massive stars form in stellar clusters where the mean stellar mass is of order 0.5M , the fragmentation mass must be of that order.…”