Radiative cooling implementations in simulations of primordial star formation

TL;DR

This study investigates how different methods of calculating radiative cooling affect the thermal evolution and collapse dynamics of primordial star-forming gas clouds in cosmological simulations.

Contribution

It compares detailed radiative transfer methods with simplified fitting functions, highlighting the importance of accurate cooling rate calculations in primordial star formation models.

Findings

Fitting functions accelerate gravitational collapse.

Hydrodynamic quantities influence cooling rates significantly.

Accurate radiative transfer methods are essential for realistic simulations.

Abstract

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.