  Many questions in physical cosmology regarding the thermal history of the
intergalactic medium, chemical enrichment, reionization, etc. are thought to be
intimately related to the nature and evolution of pregalactic structure. In
particular the efficiency of primordial star formation and the primordial IMF
are of special interest. We present results from high resolution
three--dimensional adaptive mesh refinement simulations that follow the
collapse of primordial molecular clouds and their subsequent fragmentation
within a cosmologically representative volume. Comoving scales from 128 kpc
down to 1 pc are followed accurately. Dark matter dynamics, hydrodynamics and
all relevant chemical and radiative processes (cooling) are followed
self-consistently for a cluster normalized CDM structure formation model.
Primordial molecular clouds with ~10^5 solar masses are assembled by mergers of
multiple objects that have formed hydrogen molecules in the gas phase with a
fractional abundance of ~10^-4. As the subclumps merge cooling lowers the
temperature to ~200 Kelvin in a `cold pocket' at the center of the halo. Within
this cold pocket, a quasi-hydrostatically contracting core with ~200 solar mass
and number densities > 10^5 cm^-3 is found. We find that less than 1% of the
primordial gas in such small scale structures cools and collapses to
sufficiently high densities to be available for primordial star formation.
Furthermore, it is worthwhile to note that this study achieved the highest
dynamic range covered by structured adaptive mesh techniques in cosmological
hydrodynamics to date.
