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

acc = Lres 50 AU, where:
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⊙.
We check for rotational support by comparing the specific angular mo-
mentum of the SPH particle, jSPH = vrotd, with the requirement for centrifugal
support, jcent = √ GMsinkracc, where vrot and d are the rotational velocity and
distance of the particle relative to the sink. Once the sink is formed, any SPH
particle that satisfies d < racc and jSPH < jcent is accreted onto the sink. A sink
particle can also be merged with another sink particle if these same criteria
are met. When the sink is first formed, and after each subsequent accretion
event, its position and velocity are set to the mass-weighted average of the
particles it has accreted. In this way sink particles can grow and accrete mass
over time.
As discussed in Bromm et al. (2002) and Stacy et al. (2010), our criteria
for sink formation should be robust. A gas particle must collapse two orders
of magnitude above the average density of the surrounding disk, 10 10 cm −3 ,
before it is above the density threshold for sink formation. This along with
the small value for racc and the further accretion criterion of non-rotational
support ensures that sinks are indeed formed from gravitationally collapsing
gas.
Sink particles are held at a constant density of nmax = 10 12 cm −3 , a
constant temperature of 650 K, and a constant pressure corresponding to its
temperature and density. Giving the sink a temperature and pressure prevents
101
,