  The collapse of marginally Jeans unstable primordial gas clouds in the
presence of UV radiation field is discussed. Assuming that the dynamical
collapse proceeds approximately in an isothermal self-similar fashion, we
investigate the thermal evolution of collapsing central core until H$_2$
cooling dominates photoheating and the temperature drops to below $10^4$K.
Consequently, the mass of the cooled core is evaluated as $M_{cool}=3.6\times
10^6 M_\odot(I_{21}/1)^{-0.32}$. This scale depends only on the incident UV
intensity, and provides a lower limit to the mass of collapsed objects in the
UV radiation field.
