Lithium hydride in the early Universe and in protogalactic clouds

We examine the processes of formation and destruction of LiH molecules in theprimordial gas at temperatures T \leq 5000 K in the framework of standardFriedmann cosmological models and we compute the optical depth of the Universedue to Thomson scattering of cosmic background photons on LiH. With the help ofa simple model of evolution of a spherical density perturbation we follow thelinear growth and the subsequent collapse of protogalactic clouds of variousmasses, evaluating consistently their physical characteristics and chemicalcomposition. We determine the expected level of anisotropy of the cosmicbackground radiation due to the presence of LiH molecules in high-redshiftprotogalactic clouds. We conclude that LiH spectral features generated in theprimordial gas are hardly detectable with current techniques andinstrumentation.Comment: 10 pages (LaTeX), 7 Figures to be published in Monthly Notices of the Royal Astronomical Societ