  Observations indicate that outflows from massive young stars are more
collimated during their early evolution compared to later stages. Our paper
investigates various physical processes that impacts the outflow dynamics, i.e.
its acceleration and collimation. We perform axisymmetric MHD simulations
particularly considering the radiation pressure exerted by the star and the
disk. We have modified the PLUTO code to include radiative forces in the
line-driving approximation. We launch the outflow from the innermost disk
region (r < 50 AU) by magneto-centrifugal acceleration. In order to disentangle
MHD effects from radiative forces, we start the simulation in pure MHD, and
later switch on the radiation force. We perform a parameter study considering
different stellar masses (thus luminosity), magnetic flux, and line-force
strength. For our reference simulation - assuming a 30 Msun star, we find
substantial de-collimation of 35 % due to radiation forces. The opening angle
increases from 20 deg to 32 deg for stellar masses from 20 Msun to 60 Msun. A
small change in the line-force parameter 'alpha' from 0.60 to 0.55 changes the
opening angle by ~ 8 deg. We find that it is mainly the stellar radiation which
affects the jet dynamics. Unless the disk extends very close to the star, its
pressure is too small to have much impact. Essentially, our parameter runs with
different stellar mass can be understood as a proxy for the time evolution of
the star-outflow system. Thus, we have shown that when the stellar mass (thus
luminosity) increases (with age), the outflows become less collimated.
