  We use three-dimensional hydrodynamic simulations to investigate the effects
of a transient photoionizing ultraviolet (UV) flux on the collapse and cooling
of pregalactic clouds. These clouds have masses in the range 10^5 -10^7 M_sun,
form at high redshifts (z>18), are assumed to lie within the short-lived
cosmological HII regions around the first generation of stars. In addition, we
study the combined effects of this transient UV flux and a persistent
Lyman-Werner (LW) background from distant sources. In the absence of a LW
background, we find that a critical specific intensity of J_UV ~ 0.1 x 10^-21
ergs s^-1 cm^-2 Hz^-1 sr^-1 demarcates a transition from net negative to
positive feedback for the halo population. A weaker UV flux stimulates
subsequent star formation inside the fossil HII regions, by enhancing the H_2
molecule abundance. A stronger UV flux significantly delays star-formation by
reducing the gas density, and increasing the cooling time, at the centers of
collapsing halos. At a fixed J_UV, the sign of the feedback also depends
strongly on the density of the gas at the time of UV illumination. Regardless
of the whether the feedback is positive or negative, we find that once the UV
flux is turned off, its impact stars to diminish after ~30% of the Hubble time.
In the more realistic case when a LW background is present, with J_LW > 0.01 x
10^-21 ergs s^-1 cm^-2 Hz^-1 sr^-1, strong suppression persists down to the
lowest redshift (z=18) in our simulations. Finally, we find evidence that
heating and photoevaporation by the transient UV flux renders the ~10^6 M_sun
halos inside fossil HII regions more vulnerable to subsequent H_2
photo-dissociation by a LW background.
