Temperature evolution for different values of the initial H₂ densities. The lines represent the time evolution in the cases H₂ = 10^-10 (solid line), H₂ = 10^-6 (dotted line), H₂ = 10^-2 (dashed line), H₂ = 10^-1 (dashed-dotted line), and finally, H₂ = 1 (three dots-dashed line). All the densities are in cm^-3. Top panel: the models with Z = 10^-10. Bottom panel: the same as in the top panel but for Z = 1. See the text for more details.