solar masses, which indicates that all small scale fluctuations in this
model collapse nearly simultaneously in time. This leads to very complex
dynamics during the formation of these first structures. Furthermore, the
cooling in these fluctuations is dominated by the rotational/vibrational modes
of hydrogen molecules that were able to form using the free electrons left
over from recombination and those produced by strong shock waves as
catalysts. The first structures to collapse may be capable of producing
pop III stars and have a substantial influence on
the subsequent thermal evolution of the intergalactic medium,
as suggested by Figure 1, due to the
radiation emitted by the first generation stars
as well as supernova driven winds. To know the
subsequent fate of the Universe and which structures will survive or be
destroyed by the UV background, it is first necessary to know when
and how the first stars formed.
Ostriker and Gnedin [101] have carried out high resolution numerical
simulations of the reheating and reionization of the Universe due to
star formation bursts triggered by molecular hydrogen cooling.
Accounting for the chemistry of the primeval hydrogen/helium plasma,
self-shielding of the gas, radiative cooling, and a phenomenological
model of star formation, they find that two distinct star
populations form: the first generation pop III from 
cooling
prior to reheating at redshift 
; and the second generation
pop II at z<10 when the virial temperature of the gas clumps
reaches 
K and hydrogen line cooling becomes efficient.
Star formation slows in the intermittent epoch due to the
depletion of by photo-destruction and reheating.
In addition, the objects which formed pop III stars also initiate pop II
sequences when their virial temperatures reach K
through continued mass accretion.
In resolving the details of a single star forming region in a CDM Universe,
Abel et al. [2, 3
]
implemented a non-equilibrium radiative cooling
and chemistry model [1
, 19] together wi
th the hydrodynamics
and dark matter equations, evolving nine separate atomic and molecular species
(H, H 
, He, He , He 
, H 
, H 
, H 
, and e )
on nested and adaptively refined numerical grids.
They follow the collapse and fragmentation of primordial clouds
over many decades in mass and spatial dynamical range,
finding a core of mass 
forms from a halo of about
(where a significant number fraction of hydrogen
molecules are created) after less than one percent of the halo gas
cools by molecular line emission.
Bromm et al. [43] use a different Smoothed Particle
Hydrodynamics (SPH) technique and a six species model
(H, H , H , H , H , and e )
to investigate the initial mass function
of the first generation pop III stars. They evolve an isolated
peak of mass 
which collapses at
redshift 
and forms clumps of mass 
which then grow by accretion and merging, suggesting
that the very first stars were massive and in agreement with [3].
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Computational Cosmology: from the Early Universe to the Large Scale Structure Peter Anninos http://www.livingreviews.org/lrr-2001-2 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |