Revisiting the effect of H2, HD and LiH molecules in the cooling of primordial gas

Details Revisiting the effect of H2, HD and LiH molecules in the cooling of primordial gas Revisiting the effect of H2, HD and LiH molecules in the cooling of primordial gas

2008-09-16

arxiv.org/abs/0809.2786v2

Astronomy and Astrophysics

Astrophysics

Scientific paper

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.

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