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

expected that some of the gas within these cores will gravitationally collapse
into one or more protostars of initial mass ∼ 5 × 10 −3 M⊙ (Omukai and
Nishi 1998; Yoshida et al. 2008). A fraction of the core mass will be accreted
onto the protostar(s), ultimately forming stars much more massive than the
initial seeds. Three-dimensional simulations which studied the growth of a
primordial protostar through accretion imply that the first stars were quite
massive, finding an upper limit of ∼ 500 M⊙ (Bromm and Loeb 2004). Some
of the earliest studies of Pop III star formation, however, predicted that Pop
III stellar masses might be quite low, < 1 M⊙ (e.g. Kashlinsky and Rees
1983). These authors emphasized the importance of angular momentum in
determining the mass of Pop III stars, predicting that rotational effects would
cause the primordial gas clouds to collapse into a dense disk. Only after the
disk cooled to ∼ 1000 K through H2 line emission could fragmentation occur.
Depending on the spin of the cloud, the fragments thus formed could have
had masses as low as < ∼ 0.2 M⊙. Later studies by Saigo et al. (2004) also
found that high initial angular momentum would allow primordial clouds to
form disks, and they further predicted that disk fragmention into binaries
would be common. Similar calculations by Machida et al. (2008), which were
initialized at much lower densities and therefore at an earlier point in the gas
collapse, found fragmentation would occur even for primordial gas clouds with
very modest initial angular momentum. Finally, the recent work by Clark
et al. (2008) included a simulation of a 500 M⊙ primordial gas cloud which
fragmented into ∼ 20 stellar objects.
However, none of the above-mentioned investigations were initialized
on cosmological scales. Our goal in this study is to further improve our under-
standing of how a Pop III star is assembled through the process of accretion
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