  The formation of massive stars is a long standing problem. Although a number
of theories of massive star formation exist, ideas appear to converge to a
disk-mediated accretion scenario. Here we present radiative hydrodynamic
simulations of a star accreting mass via a disk embedded in a torus. We use a
Monte Carlo based radiation hydrodynamics code to investigate the impact that
ionizing radiation has on the torus. Ionized regions in the torus midplane are
found to be either gravitationally trapped or in pressure driven expansion
depending on whether or not the size of the ionized region exceeds a critical
radius. Trapped Hii regions in the torus plane allow accretion to progress,
while expanding Hii regions disrupt the accretion torus preventing the central
star from aggregating more mass, thereby setting the star's final mass. We
obtain constraints for the luminosities and torus densities that lead to both
scenarios.
