Primordial star formation - The role of molecular hydrogen

We investigate the thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas. The cloud is assumed to be in pressure-free collapse. Over a wide range of initial conditions, virtually all the gas is converted to molecular form by a density n = 10/sup 12/ cm/sup -3/. The most effective reactions are the three-body ones: H+H+H..-->..H/sub 2/+H and H+H+H/sub 2/..-->..2H/sub 2/. As a result of significant cooling from the molecules, the temperature rise is slowed, and the Jeans mass eventually falls below 0.1 M/sub sun/ for clouds less massive than 100 M/sub sun/. Such clouds should therefore be capable of fragmenting into low-mass stars. This conclusion is strengthened if angular momentum slows the collapse. We also include in a heuristic manner the effect of shock heating from colliding fragments in a turbulent collapsing cloud. With substantial heating, the Jeans mass cannot drop as far, owing to the early destruction of hydrogen molecules. The primordial stellar mass spectrum may therefore be a sensitive function of the degree and effectiveness of intercloud collisions.