  We investigate the radiation pressure feedback in the formation of massive
stars in 1, 2, and 3D radiation hydrodynamics simulations of the collapse of
massive pre-stellar cores. In contrast to previous research, we consider
frequency dependent stellar radiation feedback, resolve the dust sublimation
front in the vicinity of the forming star down to 1.27 AU, compute the
evolution for several 10^5 yrs covering the whole accretion phase of the
forming star, and perform a comprehensive survey of the parameter space. The
most fundamental result is that the formation of a massive accretion disk in
slowly rotating cores preserves a high anisotropy in the radiation field. The
thermal radiation escapes through the optically thin atmosphere, effectively
diminishing the radiation pressure feedback onto the accretion flow.
Gravitational torques in the self-gravitating disk drive a sufficiently high
accretion rate to overcome the residual radiation pressure. Simultaneously, the
radiation pressure launches an outflow in the bipolar direction, which grows in
angle with time and releases a substantial fraction of the initial core mass
from the star-disk system. Summarized, for an initial core mass of 60, 120,
240, and 480 Msol these mechanisms allow the star to grow up to 28.2, 56.5,
92.6, and at least 137.2 Msol respectively.
