Revisiting the effect of H2, HD and LiH molecules in the cooling of primordial gas
Abstract
We use a non-equilibrium chemical network to revisit and study the effect of H2, 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≈ 1000K and a gas number density in the range ntot=1-104 cm-3. At low densities, ntot<102 cm-3, the gas reaches temperatures 100K and the main coolant is H2, but at higher densities, ntot>102 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 nD/nH=10-7-10-4.5, with nHD/nH=D+/nH=10-10. The simulations show that at ntot>102 cm-3 the HD cooling dominates the temperature evolution for D abundances greater than 10-5nH. 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 nH+/nH=10-4-10-1 and nD+/nH+=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 H2 and HD molecules, which are the main coolers at low temperatures. Finaly, we studied the effect of an OB star, with Teff=4× 104K, would have on gas cooling. It is very difficult for a gas with ntot in the range between 1-100 cm-3 to drop its temperature if the star is at a distance less than 100 pc.