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

convenient way to directly measure the accretion rate onto the protostellar
region.
3.2.4 Ray-tracing Scheme
Once the first sink is formed, this particle is used as the source of
protostellar LW and ionizing radiation. While the protostar is less massive
than 10 M⊙, LW radiation is the only source of feedback. After the protostar
is massive enough to emit ionizing radiation, however, a compact H ii region
develops. We model the growth of the surrounding I-front using a ray-tracing
scheme which closely follows that of Greif et al. (2009). A spherical grid with
∼ 10 5 rays and 200 radial bins is then created around the sink particle. The
minimum radius is determined by the distance between the sink and its closest
neighboring SPH particle, and we update this structure each time the ray-
tracing is performed. Because the sink accretes most particles within racc = 50
AU, the minimum radius is usually > ∼ 50 AU. The maximum radius is chosen
as 14 kpc (physical), a value that easily encompasses the entire H ii region
in our simulation. The radial bins within 75 AU of the minimum radius are
spaced at intervals of 1.5 AU. Outside this distance the bins are logarithmically
spaced. The location of each particle is then mapped onto the corresponding
bin within the spherical grid, and each particle contributes its density and
chemical abundances to the bin proportional to its density squared.
Next, the ionization front equation is solved along each ray:
nnr 2 I
drI
dt
= ˙Nion
4π
− αB
rI
0
nen+r 2 dr , (3.1)
where nn, ne, and n+ are the number densities of neutral particles, electrons,
and positively charged ions, respectively. The location of the ionization front
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