  Simulations predict that the first stars in an LCDM universe formed at
redshifts z>20 in minihalos with masses of about 10^6 M_sun. We have studied
their radiative feedback by simulating the propagation of ionization fronts
(I-fronts) created by these first Population III stars (M_* = 15-500 M_sun) at
z=20, within the density field of a cosmological simulation of primordial star
formation, outward thru the host minihalo and into the surrounding gas. A
three-dimensional ray-tracing calculation tracks the I-front once the H II
region evolves a "champagne flow'' inside the minihalo, after the early D-type
I-front detaches from the shock and runs ahead, becoming R-type. We take
account of the hydrodynamical back-reaction by an approximate model of the
central wind. We find that the escape fraction of ionizing radiation from the
host halo increases with stellar mass, with 0.7 < f_esc <0.9 for 80 < M_*/M_sun
< 500. To quantify the ionizing efficiency of these stars as they begin cosmic
reionization, we find that, for M_* > 80 M_sun, the ratio of gas mass ionized
to stellar mass is ~60,000, roughly half the number of ionizing photons
released per stellar baryon. Nearby minihalos are shown to trap the I-front, so
their centers remain neutral. This is contrary to the recent suggestion that
these stars would trigger formation of a second generation by fully ionizing
neighboring minihalos, stimulating H_2 formation in their cores. Finally, we
discuss how the evacuation of gas from the host halo reduces the growth and
luminosity of "miniquasars'' that may form from black hole remnants of the
first stars.
