Collapse of primordial clouds — I

We present studies of the collapse of purely baryonic Population III objects with masses ranging from 10 to 106M⊙. A spherical Lagrangian hydrodynamic code has been written to study the formation and evolution of primordial clouds, from the beginning of the recombination era (zrec ∼ 1500) until the redshift when the collapse occurs. All the relevant processes are included in the calculations, as well as the expansion of the Universe. As initial condition we take different values for the Hubble constant and for the baryonic density parameter (considering however a purely baryonic Universe), as well as different density perturbation spectra, in order to see their influence on the behaviour of the evolution of the Population III objects. We find, for example, that the first mass that collapses is 8.5 × 104 M⊙ for h = 1, Ω = 0.1 and δi = δρ/ρ = (M/M0)−1/3(1 + zrec)−1, with the mass-scale M 0 = 1015M⊙. For M0 = 4 × 1017 M⊙ we obtain 4.4 × 104 M⊙ for the first mass that collapses. The cooling–heating and photon drag processes have a key role in the collapse of the clouds and in their thermal history. Our results show, for example, that when we disregard the Compton cooling–heating, the collapse of the objects with masses >8.5 × 104 M⊙ occurs earlier. On the other hand, when we disregard the photon drag process, the collapse occurs at a higher redshift.