Copyright by Athena Ranice Stacy 2011 - The University of Texas at ...

Lres 0.5
1/3 Mres
ρmax
with ρmax nmaxmH and mH being the proton mass. The sink particle’s mass,
Msink, is initially close to the resolution mass of the simulation, Mres 0.7
M⊙. Sink particles are held at a constant density and temperature of nmax =
10 12 cm −3 and 650 K, such that their pressure is also kept to the correspond-
ing value. Providing the sink with a temperature and pressure prevents the
existence of a pressure deficit around the sink, which would otherwise lead
to artificially high accretion rates (e.g. Bate et al. 1995; Bromm et al. 2002;
Martel et al. 2006). The sink particles do continue to evolve in position and
velocity, however, through gravitational and hydrodynamic interactions.
As discussed in Bromm et al. (2002) and Stacy et al. (2010), our sink
formation criteria well represent regions that will truly collapse to stellar den-
sities. Before crossing the density threshold to become a sink, a gas particle
must collapse two orders of magnitude above the average disk density, 10 10
cm −3 . Our high density threshold and small value for racc ensure that sinks
are formed only from gravitationally collapsing gas.
Following the long-term evolution of the star-forming gas would not
be feasible without the sink particle method. By preventing gas evolution
to ever higher densities, we avoid the problem of increasingly small numeri-
cal timesteps, a problem also known as ‘Courant myopia.’ We thus are able
to see how the surrounding region of interest evolves over many dynamical
times. With sink particles, we can furthermore bypass the need to incorporate
the chemistry, hydrodynamics and radiative transfer that comes into play at
extremely high densities (n > 10 12 cm −3 ). Finally, sink particles provide a
60
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