Collapse of Low Mass Clouds in the Presence of UV Radiation Field

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.