  The most massive stars can form via standard disk accretion - despite of the
radiation pressure generated - due to the fact that the massive accretion disk
yields a strong anisotropy in the radiation field, releasing most of the
radiation pressure perpendicular to the disk accretion flow. Here, we analyze
the self-gravity of the forming circumstellar disk as the potential major
driver of the angular momentum transport in such massive disks responsible for
the high accretion rates needed for the formation of massive stars. For this
purpose, we perform self-gravity radiation hydrodynamics simulations of the
collapse of massive pre-stellar cores. The formation and evolution of the
resulting circumstellar disk is investigated in 1.) axially symmetric
simulations using an alpha-shear-viscosity prescription and 2.) a
three-dimensional simulation, in which the angular momentum transport is
provided self-consistently by developing gravitational torques in the
self-gravitating accretion disk. The simulation series of different strength of
the alpha-viscosity shows that the accretion history of the forming star is
mostly independent of the alpha-viscosity-parameter. The accretion history of
the three-dimensional run driven by self-gravity is more time-dependent than
the viscous disk evolution in axial symmetry. The mean accretion rate, i.e. the
stellar mass growth, is nearly identical to the alpha-viscosity models. We
conclude that the development of gravitational torques in self-gravitating
disks around forming massive stars provides a self-consistent mechanism to
efficiently transport the angular momentum to outer disk radii. Also the
formation of the most massive stars can therefore be understood in the standard
accretion disk scenario.
