Primordial star formation - The role of molecular hydrogen

Abstract

The thermal and chemical evolution of a collapsing spherical cloud composed of pure hydrogen gas is investigated. It is assumed that the cloud is in pressure-free collapse. Over a broad range of initial conditions, virtually all the gas is converted to molecular form by a density n = 10 to the 12th/cu cm. The reactions found to be most effective are the three-body ones: H + H + H yielding H2 + H; and H + H + H2 yielding 2H2. As a consequence of significant cooling from the molecules, the temperature rise is slowed, and the Jeans mass eventually falls below 0.1 solar mass for clouds less massive than 100 solar masses. Such clouds should thus be capable of fragmenting into low-mass stars. This conclusion is even more valid if angular momentum slows the collapse. Also included in a heuristic manner is the effect of shock heating from colliding fragments in a turbulent collapsing cloud. Owing to the early destruction of hydrogen molecules, the Jeans mass cannot drop as far with substantial heating.The primordial stellar mass spectrum may thus be a sensitive function of the degree and effectiveness of intercloud collisions.