Formation of Primordial Protostars

Abstract

The evolution of collapsing metal-free protostellar clouds is investigated for various masses and initial conditions. We perform hydrodynamical calculations for spherically symmetric clouds, taking into account radiative transfer of the molecular hydrogen lines and the continuum as well as the chemistry of molecular hydrogen. The collapse is found to proceed almost self-similarly, like the Larson-Penston similarity solution. In the course of the collapse, efficient three-body processes transform atomic hydrogen in an inner region of ~1 M ☉ entirely into molecular form. However, hydrogen in the outer part remains totally atomic, although there is an intervening transitional layer of several solar masses, where hydrogen is in partially molecular form. No opaque transient core is formed, although clouds become optically thick to H 2 collision-induced absorption continuum, since H 2 dissociation follows successively. When the central part of the cloud reaches stellar densities (~10 -2 g cm -3 ), a very small hydrostatic core (~5 × 10 -3 M ☉ ) is formed and subsequently grows in mass as the ambient gas accretes onto it. The mass accretion rate is estimated to be 3.7 × 10 -2 M ☉ yr -1 ( M * / M ☉ ) -0.37 , where M * is the instantaneous mass of the central core, by using a similarity solution that reproduces the evolution of the cloud before the core formation.