Molecules in the early universe

Calculations are presented of the formation of astrophysically interesting molecules (H2, HD, LiH, and HeH+) by gas-phase reactions during the postrecombination epoch (redshifts z = 300-30). In standard Friedmann cosmological models, H2/H is approximately equal to 10 to the -6th, HD/H2 is approximately equal to 10 to the -4.5th, and LiH/H2 is approximately equal to 10 to the -6.5th. These molecules may dominate the cooling and trigger the collapse of primordial gas clouds. The dipole rotational transitions of HD and LiH are particularly important at high density and low temperature. Additional molecules form during spherical collapse of these clouds, and their rotational cooling keeps the gas temperature between 400 and 1500 K over 12 decades of density increase until the H2 lines becomes optically thick. The existence of molecular coolants at high redshift has significant implications for the first generation of stars and for thermal instabilities in intergalactic matter.