Radiation from the first forming stars

The evolution of radiation emitted during the dynamical collapse of metal-free protostellar clouds is investigated within a spherically symmetric hydrodynamical scheme that includes the transfer of radiation and the chemistry of the primordial gas. The cloud centre collapses on a time-scale of similar to10(5-6) yr, thanks to line cooling from molecular hydrogen (H-2). For most of the collapse time, when the evolution proceeds self-similarly, the luminosity slowly rises up to similar to10(36) erg and is essentially a result of H-2 infrared (IR) line emission. Later, continuum IR radiation provides an additional contribution, which is mostly a result of the accretion of an infalling envelope upon a small hydrostatic protostellar core that develops in the centre. We follow the beginning of the accretion phase, when the enormous accretion rate (similar to0.1 M-circle dot yr(-1)) produces a very high continuum luminosity of similar to10(36) erg. Despite the high luminosities, the radiation field is unable to affect the gas dynamics during the collapse and the first phases of accretion, because the opacity of the infalling gas is too small; this is quite different from present-day star formation. We also find that the protostellar evolution is similar among clouds with different initial configurations, including those resulting from three-dimensional cosmological simulations of primordial objects; in particular, the shape of the molecular spectra is quite universal. Finally, we briefly discuss the detectability of this initial cosmic star formation activity.