We use a non-equilibrium chemical network to revisit and study the effect of H_{ 2}, HD and LiH molecular cooling on a primordial element of gas. We solve both the thermal and chemical equations for a gas element with an initial temperature T\approx 1000K and a gas number density in the range n_{ tot}=1-10^{ 4} cm^{ -3}. At low densities, n_{ tot}<10^{ 2} cm^{ -3}, the gas reaches temperatures \sim 100K and the main coolant is H_{ 2}, but at higher densities, n_{ tot}>10^{ 2} cm^{ -3}, the HD molecule dominates the gas temperature evolution. The effect of LiH is negligible in all cases. We studied the effect of D abundance on the gas cooling. The D abundance was set initially to be in the range n_{ D}/n_{ H}=10^{ -7}-10^{ -4.5}, with n_{ HD}/n_{ H}={ D^{ +}}/n_{ H}=10^{ -10}. The simulations show that at n_{ tot}>10^{ 2} cm^{ -3} the HD cooling dominates the temperature evolution for D abundances greater than 10^{ -5}n_{ H}. This number decrease at higher densities. Furthermore, we studied the effect of electrons and ionized particules on the gas temperature. We followed the gas temperature evolution with n_{ H_{ +}}/n_{ H}=10^{ -4}-10^{ -1} and n_{ D^{ +}}/n_{ H^{ +}}=10^{ -5}. The gas temperature reached lower values at high ionization degree because electrons, H^{ +} and D^{ +} are catalizers in the formation paths of the H_{ 2} and HD molecules, which are the main coolers at low temperatures. Finaly, we studied the effect of an OB star, with T_{ eff}=4\times 10^{ 4}K, would have on gas cooling. It is very difficult for a gas with n_{ tot} in the range between 1-100 cm^{ -3} to drop its temperature if the star is at a distance less than 100 pc.
Categories: astro-ph