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abstract
The thermo-chemical equilibrium of a homogeneous cloud constituted by primordial gas has been calculated as a function of the particle density and taking as free parameters: the ionizing and the dissociating external flux and the mass of the cloud. The cloud is assumed to be composed by a gas with nine chemical species: $H$, $H_{2}$, $H^{+}$, $e^{-}$, $H^{-}$, $H_{2}^{+}$, $H_{e}$, $H_{e}^{+}$, and $H_{e}^{++}$. A total of 24 chemical reactions are considered. These reactions include processes of photo-ionization, photo-dissociation, recombination, and charge exchange. The equilibrium is calculated considering heating rates due to collisional excitation of atomic and molecular species. There are two ranges of density where the temperature becomes a many-valued function of the density. The results suggest that for $M \sim 10^{9} M\odot$, the onset of the gravitational collapse could be triggered by the effects of the recombination of the hydrogen. Additionally, the threshold values of the ionizing and dissociating external flux beyond which a cooling primordial gas cloud reaches thermal equilibrium at a temperature near $10^{4}$ K are calculated. Applications of the above results to the Ly-alpha cloud structure and to the problem of globular clusters formation are outlined.
