Radiative cooling implementations in simulations of primordial star
  formation
  We study the thermal evolution of primordial star-forming gas clouds using
three-dimensional cosmological simulations. We critically examine how
assumptions and approximations made in calculating radiative cooling rates
affect the dynamics of the collapsing gas clouds. We consider two important
molecular hydrogen cooling processes that operate in a dense primordial gas;
H_2 line cooling and continuum cooling by H_2 collision-induced emission. To
calculate the optically thick cooling rates, we follow the Sobolev method for
the former, whereas we perform ray-tracing for the latter. We also run the same
set of simulations using simplified fitting functions for the net cooling
rates. We compare the simulation results in detail. We show that the time- and
direction-dependence of hydrodynamic quantities such as gas temperature and
local velocity gradients significantly affects the optically thick cooling
rates. Gravitational collapse of the cloud core is accelerated when the cooling
rates are calculated by using the fitting functions. The structure and
evolution of the central pre-stellar disk are also affected. We conclude that
physically motivated implementations of radiative transfer are necessary to
follow accurately the thermal and chemical evolution of a primordial gas to
high densities.
