Collapse of primordial clouds - I

We present studies of the collapse of purely baryonic Population III object s with masses ranging from 10 to 10(6) M.. A spherical Lagrangian hydrodyna mic code has been written to study the formation and evolution of primordia l clouds, from the beginning of the recombination era(z(rec) similar to 150 0) 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 U niverse), as well as different density perturbation spectra, in order to se e their influence on the behaviour of the evolution of the Population III o bjects. We find, for example, that the first mass that collapses is 8.5 x 1 0(4) M. for h = 1, Omega = 0.1 and delta(i) = delta rho/rho = (M/M-0)(-1/3) (1+z(rec))(-1), with the mass-scale M-0 = 10(15) M.. For M-0 = 4 x 10(17) M . we obtain 4.4 x 10(4) M. for the first mass that collapses. The cooling-h eating and photon drag processes have a key role in the collapse of the clo uds 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 x 104 M. occurs earlier. On the other hand, when we disregard the photon drag process, the collapse occurs at a higher redshift.