Evolution of Primordial Protostellar Clouds

Extract

The formation of stars in proto-galactic clouds can be viewed as two step processes i.e. the fragmentation of proto-galactic clouds and evolution of these fragments into stars. We consider here the latter process, the contraction of protosteller clouds (∼ 1M_⊙) which consist of primordial gas. We investigate cooling processes by calculating the radiative transfer of H₂ rotational/vibrational lines. We consider clouds in hydrostatic equilibrium as initial conditions. Comparing two timescales, the freefall time and the timescale of quasi-static contraction (∼ t_cool, the cooling time) of these clouds, we find that as the clouds contract, the ratio of two timescales t_ff/t_qsc, i.e. the efficiency of cooling, becomes larger even under the existence of cold and opaque envelope. Especially for the fragments of primordial filamentary clouds (t_ff ∼ t_qsc initially), they collapse dynamically in the freefall timescale. This efficiency of cooling is utterly peculiar to the line cooling.