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

actual final mass of the star is likely to be significantly lower. Furthermore,
feedback effects will likely slow accretion, or even halt it entirely, before the
star dies. Thus, our extrapolation serves as a robust upper limit to the final
Pop III stellar mass.
2.4.2.5 Thermodynamics of accretion flow
After the initial sink particle has grown in mass, the surrounding gas
divides into two phases - a hot and cold one. In Fig. 2.10, we illustrate this
bifurcation with a temperature-density phase diagram at various stages of the
simulation. Heating becomes significant once the initial sink grows beyond 10
M⊙. At this mass the gravitational force of the sink particle is strong enough
to pull gas towards it with velocites sufficiently high to heat the gas to a
maximum temperature of ∼ 7,000 K (see discussion in Chapter 2.4.2.1). This
is the temperature where the collisional excitation cooling of atomic hydrogen
begins to dominate over the adiabatic and viscous heating. The increasing
mass of the sink causes a pressure wave to propagate outward from the sink
and heat particles at progressively larger radii and lower density, as is visible
in Fig. 2.5 and Fig. 2.10. The heated region extends out to ∼ 2,000 AU at
5000 yr. The pressure wave thus propagates at a subsonic speed of ∼ 2 km s −1 .
The main sink is able to accrete a fraction of these heated particles
(e.g. the yellow particle in Fig. 2.10), while it continues to accrete cold parti-
cles from the disk as well. We record the temperatures of the SPH particles
accreted onto the sink, so that we can track the relative contributions from
the two phases. When the main sink has grown to ∼ 30 M⊙, accretion from
the hot phase begins to occur, such that heated particles contribute slightly
over 50% to the total rate towards the end of the simulation. By this time,
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