  We calculate numerically the collapse of slowly rotating, non-magnetic,
massive molecular clumps, which conceivably could lead to the formation of
massive stars. Because radiative acceleration on dust grains plays a critical
role in the clump's dynamical evolution, we utilize a wavelength-dependent
radiation transfer and a three component dust model: amorphous carbon
particles, silicates and "dirty ice"-coated silicates. We do not spatially
resolve the innermost regions of the molecular clump and assume that all
material in the innermost grid cell accretes onto a single object. We introduce
a semi-analytical scheme for augmenting existing evolution tracks of pre-main
sequence protostars by including the effects of accretion. By considering an
open outermost boundary, an arbitrary amount of material could, in principal,
be accreted onto this central star. However, for the three cases considered
(30, 60, and 120 solar masses originally within the computation grid),
radiation acceleration limited the final masses to 31.6, 33.6, and 42.9 solar
masses, respectively, for wavelength-dependent radiation transfer and to 19.1,
20.1, and 22.9 solar masses for comparison simulations with grey radiation
transfer. We demonstrate that massive stars can in principle be formed via
accretion through a disk. We conclude with the warning that a careful treatment
of radiation transfer is a mandatory requirement for realistic simulations of
the formation of massive stars.
